Valentia Island () is one of Ireland's most westerly points. It lies in Dingle Bay off the Iveragh Peninsula in the southwest of County Kerry. It is linked to the mainland by the Maurice O'Neill Memorial Bridge at Portmagee. A car ferry also departs from Reenard Point to Knightstown, the island's main settlement, from April to October. Another, smaller village named Chapeltown sits at roughly the midpoint of the island, from the bridge. Valentia Island's permanent population is 658 (). It was anglicised as 'Bealinche' and 'Ballentia' before evolving into 'Valentia'. It is possible the spelling was influenced by Spanish sailors; there is a grave marker to Spanish sailors lost at sea in the Catholic cemetery at Kylemore.
The Irish name for the island is Dairbhre, probably meaning "oak isle". It was historically anglicized as 'Dariry'. the memorial marks the importance of the site to telegraph communications with North America from 1857 forward and to accurately linking longitude measurements in North America to the Royal Observatory in Greenwich in 1866.]]
Valentia was the eastern terminus of the first commercially viable transatlantic telegraph cable. The first attempt in 1857 to land a cable from Ballycarbery Strand on the mainland just east of Valentia Island ended in disappointment. After subsequent failures of cables landed at Knightstown in 1858 and Foilhommerum Bay in 1865, the vast endeavor finally resulted in commercially viable transatlantic telegraph communications from Foilhommerum Bay to Heart's Content, Newfoundland in 1866. Transatlantic telegraph cables operated from Valentia Island for one hundred years, ending with Western Union International terminating its cable operations in 1966.
Before the transatlantic telegraph, American longitude measurements had a uncertainty with respect to European longitudes. Because of the importance of accurate longitudes to safe navigation, the United States Coast Survey mounted a longitude expedition in 1866 to link longitudes in the United States accurately to the Royal Observatory in Greenwich. Benjamin Gould and his partner A. T. Mosman reached Valentia on 2 October 1866. They built a temporary longitude observatory beside the Foilhommerum Cable Station to support synchronised longitude observations with Heart's Content, Newfoundland. After many rainy and cloudy days, the first transatlantic longitude signals were exchanged between Foilhommerum and Heart's Content on 24 October 1866.
The transatlantic cable is currently on the tentative list of UNESCO World Heritage sites.
On 21 May 1927, Charles A. Lindbergh made his first landfall in Europe over Dingle Bay and Valentia Island on his solo flight from New York to Paris. On the 1927 Mercator chart used by the famous pilot, it was labelled Valencia.
In 1993, an undergraduate geology student discovered fossilised tetrapod trackways, footprints preserved in Devonian rocks, on the north coast of the island at Dohilla (). About 385 million years ago, a primitive vertebrate passed near a river margin in the sub-equatorial river basin that is now southwestern Ireland and left prints in the damp sand. The prints were preserved by silt and sand overlying them, and were converted to rock over geological time. The Valentia Island trackways are among the oldest signs of vertebrate life on land.
On 14 March 2021, Valentia Island was the site of the first walrus sighting in Ireland.