thumb|Map of dioceses in medieval Ireland
The Synod of Ráth Breasail (or Rathbreasail; ) was a synod of the Catholic Church in Ireland that took place in Ireland in 1111. It marked the transition of the Irish church from a monastic to a diocesan and parish-based church. Many present-day Irish dioceses trace their boundaries to decisions made at the synod.
Background
Viking raids on Ireland began around the start of the 9th century, and had a devastating effect on the Irish church. These disruptions, along with secular impositions by the invaders, produced a decline in Christian religious observance and the moral standards established by Saint Patrick and other early missionaries. Apathy towards these Christian teachings increased, causing many parts of Ireland to return to paganism by the 11th century, weakening Christianity's grasp on the populace.
Gradually, as the onslaughts of the Danes became less frequent, there was a revival of religious education, which prepared the way for the religious reforms of the 12th century.
It was convened by the papal legate, Gille, Bishop of Limerick. Gille is not mentioned in the Irish Annals, possibly because Limerick was then a Hiberno-Norse city. Its purpose was the Romanising of the Irish Church, and, in particular, the establishment of diocesan episcopacy.
The synod was attended by no fewer than fifty bishops, three hundred priests and three thousand laymen, including King Murtough O'Brien. There were no representatives of the provinces of Connaught and Leinster, in which the Reform movement had not yet established itself.