thumb|right|Ireland in [[Europe]]

United Ireland (), also referred to as Irish reunification or a New Ireland, is the proposition that all of Ireland should be a single sovereign state. At present, the island is divided politically: the sovereign state of Ireland (legally described also as the Republic of Ireland) has jurisdiction over the majority of Ireland, while Northern Ireland, which lies entirely within (but consists of only 6 of 9 counties of) the Irish province of Ulster, is part of the United Kingdom. Achieving a united Ireland is a central tenet of Irish nationalism and Republicanism, particularly of both mainstream and dissident republican political as well as paramilitary organisations. Unionists support Northern Ireland remaining part of the United Kingdom and oppose Irish unification.

Ireland has been partitioned since May 1921, when the Government of Ireland Act 1920 came into effect, creating two separate jurisdictions—Southern Ireland and Northern Ireland—within the United Kingdom. Southern Ireland never fully functioned and was soon replaced by the Irish Free State in 1922, which became independent, while Northern Ireland opted to remain part of the UK. The Anglo-Irish Treaty, which led to the establishment in December 1922 of a dominion called the Irish Free State, recognised partition, but this was opposed by anti-Treaty republicans. When the anti-Treaty Fianna Fáil party came to power in the 1930s, it adopted a new constitution which claimed sovereignty over the entire island. The Irish Republican Army (IRA) had a united Ireland as its goal during the conflict with British security forces and loyalist paramilitaries from the 1960s to the 1990s known as The Troubles. The Good Friday Agreement signed in 1998, which ended the conflict, acknowledged the legitimacy of the desire for a united Ireland, while declaring that it could be achieved only with the consent of a majority of the people of both jurisdictions on the island, and providing a mechanism for ascertaining this in certain circumstances.

In 2016, following the United Kingdom's decision to leave the European Union with Brexit, Sinn Féin called for a referendum on Irish reunification. The decision had increased the perceived likelihood of a united Ireland, in order to avoid the possible requirement for a hard border between Northern Ireland and the Republic of Ireland, though the imposition of a hard border has not, as yet, eventuated. Fine Gael Taoiseach Enda Kenny successfully negotiated that in the event of reunification, Northern Ireland will become part of the EU, just as East Germany was permitted to join the EU's predecessor institutions by reuniting with the rest of Germany after the fall of the Berlin Wall.

The majority of Ulster Protestants, almost half the population of Northern Ireland, favour continued union with Great Britain, and have done so historically. Four of the six counties have Irish Catholic majorities, and majorities voting for Irish nationalist parties, and Catholics have become the plurality in Northern Ireland as of 2021. Religious affiliation in Northern Ireland offers only a general indication of political preference, as some Protestants support a united Ireland while some Catholics oppose it. Two surveys in 2011 identified a significant number of Catholics who favoured the continuation of the union without identifying themselves as unionists or British.

In 2024, a survey showed supporters of the union equated to a plurality at 48.6%, rather than a majority in Northern Ireland for the first time, while 33.76% supported Irish unity.

Article 3.1 of the Constitution of Ireland "recognises that a united Ireland shall be brought about only by peaceful means with the consent of a majority of the people, democratically expressed, in both jurisdictions in the island". This provision was introduced in 1999 after implementation of the Good Friday Agreement, as part of replacing the old Articles 2 and 3, which had laid a direct claim to the whole island as the national territory.

The Northern Ireland Act 1998, a statute of the Parliament of the United Kingdom, provides that Northern Ireland will remain within the United Kingdom unless a majority of the people of Northern Ireland vote to form part of a united Ireland. It specifies that the Secretary of State for Northern Ireland "shall exercise the power [to hold a referendum] if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland". Such referendums may not take place within seven years of each other.

The Northern Ireland Act 1998 supersedes previous similar legislative provisions. The Northern Ireland Constitution Act 1973 also provided that Northern Ireland remained part of the United Kingdom unless a majority voted otherwise in a referendum, while under the Ireland Act 1949 the consent of the Parliament of Northern Ireland was needed for a united Ireland. In 1985, the Anglo-Irish Agreement affirmed, while providing for devolved government in Northern Ireland, and an advisory role for the Republic of Ireland government, that any change in the status of Northern Ireland would only come about with the consent of a majority of the people of Northern Ireland.

History

Home Rule, resistance and the Easter Rising

thumb|232x232px|[[1885 United Kingdom general election|1885 general election winning party vote share by constituency (Note: The Representation of the People Act extending the vote to all men over 21 and most women over 30 did not arrive until 1918.)]]

The Kingdom of Ireland as a whole had become part of the United Kingdom of Great Britain and Ireland under the Acts of Union 1800. From the 1870s, support for some form of an elected parliament in Dublin grew. In 1870, Isaac Butt, who was a Protestant, formed the Home Government Association, which became the Home Rule League. Charles Stewart Parnell, also a Protestant, became leader in 1880, and the organisation became the Irish National League in 1882. Despite the religion of its early leaders, its support was strongly associated with Irish Catholics. In 1886, Parnell formed a parliamentary alliance with Liberal Party Prime Minister William Ewart Gladstone and secured the introduction of the First Home Rule Bill. This was opposed by the Conservative Party and led to a split in the Liberal Party, the Liberal Unionist Party. Opposition in Ireland was concentrated in the heavily Protestant counties in Ulster. The difference in religious background was a legacy of the Ulster Plantation in the early seventeenth century. In 1893, the Second Home Rule Bill passed in the House of Commons, but was defeated in the House of Lords, where the Conservatives dominated. A Third Home Rule Bill was introduced in 1912, and in September 1912, just under half a million men and women signed the Ulster Covenant to swear they would resist its application in Ulster. The Ulster Volunteer Force were formed in 1913 as a militia to resist Home Rule.

The Government of Ireland Act 1914 (previously known as the Third Home Rule Bill) provided for a unitary devolved Irish Parliament, a culmination of several decades of work from the Irish Parliamentary Party. It was signed into law in September 1914 in the midst of the Home Rule Crisis and at the outbreak of the First World War. On the same day, the Suspensory Act 1914 suspended its actual operation.thumb|[[Proclamation of the Irish Republic, presented to the Irish people during the Easter Rising of 1916|249x249px]]In 1916, a group of revolutionaries led by the Irish Republican Brotherhood launched the Easter Rising, during which they issued a Proclamation of the Irish Republic. The rebellion was not successful and sixteen of the leaders were executed. The small separatist party Sinn Féin became associated with the Rising in its aftermath as several of those involved in it were party members.

The Irish Convention held between 1917 and 1918 sought to reach agreement on manner in which Home Rule would be implemented after the war. All Irish parties were invited, but Sinn Féin boycotted the proceedings. By the end of the First World War, a number of moderate unionists came to support Home Rule, believing that it was the only way to keep a united Ireland in the United Kingdom. The Irish Dominion League opposed partition of Ireland into separate southern and northern jurisdictions, while arguing that the whole of Ireland should be granted dominion status with the British Empire.

thumb|Result of the [[1918 United Kingdom general election in Ireland|1918 Irish general election]]

At the 1918 election Sinn Féin won 73 of the 105 seats; however, there was a strong regional divide, with the Ulster Unionist Party (UUP) winning 23 of the 38 seats in Ulster. Sinn Féin had run on a manifesto of abstaining from the United Kingdom House of Commons, and from 1919 met in Dublin as Dáil Éireann. At its first meeting, the Dáil adopted the Declaration of Independence of the Irish Republic, a claim which it made in respect of the entire island. Supporters of this Declaration fought in the Irish War of Independence.

Two jurisdictions

During this period, the Government of Ireland Act 1920 repealed the previous 1914 Act, and provided for two separate devolved parliaments in Ireland. It defined Northern Ireland as "the parliamentary counties of Antrim, Armagh, Down, Fermanagh, Londonderry and Tyrone, and the parliamentary boroughs of Belfast and Londonderry" and Southern Ireland "so much of Ireland as is not comprised within the said parliamentary counties and boroughs". Section 3 of this Act provided that the parliaments may be united by identical acts of parliament:

Sinn Féin did not recognise this act, treating elections to the respective parliaments as a single election to the 2nd Dáil. While the Parliament of Northern Ireland sat from 1921 to 1972, the Parliament of Southern Ireland was suspended after its first meeting was boycotted by the Sinn Féin members, who comprised 124 of its 128 MPs.

A truce in the War of Independence was called in July 1921, followed by negotiations in London between the government of the United Kingdom and a Sinn Féin delegation. On 6 December 1921, they signed the Anglo-Irish Treaty, which led to the establishment of the Irish Free State the following year, a dominion within the British Empire.

With respect to Northern Ireland, Articles 11 and 12 of the Treaty made special provision for it including as follows:

The Prime Minister of Northern Ireland, Sir James Craig, speaking in the House of Commons of Northern Ireland in October 1922 said that "when 6 December [1922] is passed the month begins in which we will have to make the choice either to vote out or remain within the Free State". He said it was important that that choice be made as soon as possible after 6 December 1922 "in order that it may not go forth to the world that we had the slightest hesitation". On 7 December 1922, the day after the establishment of the Irish Free State, the Houses of the Parliament of Northern Ireland resolved to make the following address to the King so as to exercise the rights conferred on Northern Ireland under Article 12 of the Treaty:

The King received it the following day. These steps cemented Northern Ireland's legal separation from the Irish Free State.

In Irish republican legitimist theory, the Treaty was illegitimate and could not be approved. According to this theory, the 2nd Dáil did not dissolve and members of the Republican Government remained as the legitimate government of the Irish Republic declared in 1919. Adherents to this theory rejected the legitimacy of both the Irish Free State and Northern Ireland.

The report of Boundary Commission in 1925 established under the Treaty did not lead to any alteration in the border.

Within Northern Ireland, the Nationalist Party was an organisational successor to the Home Rule Movement, and advocated the end of partition. It had a continuous presence in the Northern Ireland Parliament from 1921 to 1972, but was in permanent opposition to the UUP government.

A new Constitution of Ireland was proposed by Éamon de Valera in 1937 and approved by the voters of the Irish Free State (thereafter simply Ireland). Articles 2 and 3 of this Constitution claimed the whole island of Ireland as the national territory, while claiming legal jurisdiction only over the previous territory of the Irish Free State.

Article 15.2 allowed for the "creation or recognition of subordinate legislatures and for the powers and functions of these legislatures", which would have allowed for the continuation of the Parliament of Northern Ireland within a unitary Irish state.

In 1946, former Prime Minister Winston Churchill told the Irish High Commissioner to the United Kingdom, "I said a few words in Parliament the other day about your country because I still hope for a United Ireland. You must get those fellows in the north in, though; you can't do it by force. There is not, and never was, any bitterness in my heart towards your country." He later said, "You know I have had many invitations to visit Ulster but I have refused them all. I don't want to go there at all, I would much rather go to southern Ireland. Maybe I'll buy another horse with an entry in the Irish Derby."

Under the Republic of Ireland Act 1948, Ireland declared that the country may officially be described as the Republic of Ireland and that the President of Ireland had the executive authority of the state in its external relations. This was treated by the British Commonwealth as ending Irish membership. In response, the United Kingdom passed the Ireland Act 1949. Section 1(2) of this act affirmed the provision in the Treaty that the position of Ireland remained a matter for the Parliament of Northern Ireland:

The political organization Irish Anti-Partition League (APL) existed in Northern Ireland from 1945-1958. One of its founding members was the political leader from Derry, Cahir Healy. In 1958 Healy wrote about the decline of the APL and the economic conditions which helped keep partition in place:

In 1956 Irelands Taoiseach (or Prime Minister) John A. Costello tasked the Department of External Affairs with preparing a document that summarized Irish policy on partition. The document advocated for efforts to win over moderate Unionists in Northern Ireland but stated bluntly that "We are no nearer to success than in 1923."

Between 1956 and 1962, the IRA engaged in a border campaign against British Army and Royal Ulster Constabulary outposts with the aim of ending British rule in Northern Ireland. This coincided with brief electoral success of Sinn Féin, which won four seats at the 1957 Irish general election. This was its first electoral success since 1927, and it did not win seats in the Republic of Ireland again until 1997. The border campaign was entirely unsuccessful in its aims. In 1957, Prime Minister Harold Macmillan wrote that "I do not think that a United Ireland – with de Valera as a kind of Irish Nehru would do us much good. Let us stand by our friends."

Calls for unification, start of the Troubles

The Northern Ireland civil rights movement emerged in 1967 to campaign for civil rights for Catholics in Northern Ireland. Tensions between republican and loyalist groups in the north erupted into outright violence in the late 1960s. In 1968 the Irish Taoiseach, Jack Lynch, raised the issue of partition in London: "It has been the aim of my government and its predecessors to promote the reunification of Ireland by fostering a spirit of brotherhood among all sections of the Irish people. The clashes in the streets of Derry are an expression of the evils which partition has brought in its train." He later stated to the press that the ending of partition would be "a just and inevitable solution to the problems of Northern Ireland."

Lynch renewed his call to end partition in August 1969 when he proposed negotiations with Britain with the hope of merging the Irish Republic and Northern Ireland into a federal type state. Lynch proposed that the two parliaments continue to function with a Council of Ireland having authority over the entire country. The Prime Minister of Northern Ireland James Chichester-Clark rejected the proposal. In August 1971 Lynch proposed that the Government of Northern Ireland (Stormont) be replaced with an administration that would share power with Catholics. The next day the Northern Prime Minister Brian Faulkner rejected Lynch's statement and stated that "no further attempt by us to deal constructively with the present Dublin government is possible." Later in 1971 British Labour Party leader (and future Prime Minister) Harold Wilson proposed a plan that would lead to a united Ireland after a 15-year transitional period. He called for the establishment of a commission that would examine the possibility of creating a united Ireland which would be agreed upon by all three parliaments. The northern Prime Minister rejected the proposal and reiterated the desire that Northern Ireland remain an integral part of the United Kingdom. The Irish Taoiseach indicated the possibility of amending the Irish constitution to accommodate the Protestants of Northern Ireland and urged the British government to "declare its interest in encouraging the unity of Ireland".

In 1969 the British government deployed troops in what would become the longest continuous deployment in British military history Operation Banner. The Provisional Irish Republican Army (IRA) had begun a thirty-year campaign against British security forces with the aim of winning a united Ireland.

In 1970, the Social Democratic and Labour Party (SDLP) was established to campaign for civil rights and a united Ireland by peaceful, constitutional means. The party rose to be the dominant party representing the nationalist community until the early twenty-first century.

In 1972, the parliament of Northern Ireland was suspended, and under the Northern Ireland Constitution Act 1973, it was formally abolished. Section 1 of the 1973 Act stated,

A border poll was held in Northern Ireland in 1973. The SDLP and Sinn Féin called for a boycott of the poll. 98.9% of votes cast supported remaining part of the United Kingdom. The poll was overwhelmingly boycotted by nationalists, and the turnout was therefore 58.7%. The pro-UK vote did however represent 57.5% of the entire electorate, notwithstanding the boycott.

In 1983, the Irish government led by Taoiseach Garret FitzGerald established the New Ireland Forum as a consultation on a new Ireland. Though all parties in Ireland were invited, the only ones to attend were Fine Gael, Fianna Fáil, the Labour Party and the SDLP. Its report considered three options: a unitary state, i.e., a united Ireland; a federal/confederal state; and joint sovereignty. These options were rejected by Prime Minister Margaret Thatcher. In 1985, the governments of Ireland and of the United Kingdom signed the Anglo-Irish Agreement; the British government accepted an advisory role for the Irish government in the future of Northern Ireland. Article 1 of the Agreement stated that the future constitutional position of Northern Ireland would be a matter for the people of Northern Ireland:

In the Downing Street Declaration, Taoiseach Albert Reynolds and Prime Minister John Major issued a joint statement, in which Major, "reiterated on behalf of the British Government, that they have no selfish strategic or economic interest in Northern Ireland".

Good Friday Agreement

thumb|169x169px|"Vote Yes, It's the way Ahead", Yes Campaign for the Belfast Agreement, 1998

The Good Friday Agreement in 1998 was a culmination of the peace process. The agreement acknowledged nationalism and unionism as "equally legitimate, political aspirations". In the Northern Ireland Assembly, all members would designate as Unionist, Nationalist, or Other, and certain measures would require cross-community support. The agreement was signed by the governments of Ireland and of the United Kingdom. In Northern Ireland, it was supported by all parties who were in the Northern Ireland Forum with the exception of the Democratic Unionist Party and the UK Unionist Party, and it was supported by all parties in the Oireachtas. It was also opposed by dissident republicans, including Republican Sinn Féin and the 32 County Sovereignty Movement. It was approved in referendums in Northern Ireland and in the Republic of Ireland.

Included in the Agreement were provisions which became part of the Northern Ireland Act 1998 on the form of a future referendum on a united Ireland. In essence the Good Friday Agreement provided the opportunity for self determination and mutual respect. Those born in Northern Ireland could identify as Irish. The freedom of movement, allowed citizens of either jurisdiction to live in which ever part of the island they wanted, thereby enabling them to choose which state they paid taxes to or claimed benefits from. The 'Two state' solution advocated for conflict resolution in other jurisdictions therefore applied. Provision within the Agreement allows for a simple majority to vote in favour of Irish Unification, but does not explain how the dissolution of the two states solution would lead to the inclusion of the unionist population into a unified Ireland in which they would constitute 13% of the population. A fear of political, civil and economic turmoil and a lack of protection for minority rights, as experienced by the Catholic community in Northern Ireland and the Protestant community in the Republic of Ireland historically, is a key driver towards the desire for the maintenance of the status quo on both sides of the border.