The Kingdom of East Breifne or Breifne O'Reilly (; , ) was an historic kingdom of Ireland roughly corresponding to County Cavan that existed from 1256 to 1607. It took its present boundaries in 1579 when East Breifne was renamed Cavan, after Cavan town, and shired into Ulster. Originally part of the older Kingdom of Breifne, East Breifne came into existence following a protracted war between the ruling O'Rourke clan and the ascendant O'Reillys which culminated in the division of the kingdom in 1256. The Kingdom was ruled by the dynasty of the Ó Raghallaigh (O'Reilly) and lasted until the early 17th century.
Origins and etymology
The area of modern-day east County Cavan has been inhabited for over 5,000 years. The O'Reilly are descendant from a kin-group known as Uí Briúin, who settled the east Breifne area in the eighth century AD. At some point they splintered off from the Uí Briúin sept and became known as Muintir-Maelmordha, named after their chief Maelmordha who lived in the 9th century. They did not assume a surname until the early 11th century when they became known as O Raghallaigh, from the chieftain Raghallach. They are believed to have arrived in what is now County Cavan in the 10th century. Muintir-Maelmordha first appear in the Irish annals as a clan ruling a small territory around Lough Ramor called Machaire Gailenga in 1126, at which point they are a vassal of the O'Rourkes.
Kingdom of Breifne c. 1120–1256
Machaire Gailenga was annexed after the O'Reilly were defeated by the O'Rourkes and became a constituent clan within what was then known as the Kingdom of Bréifne and Conmaicne. King Tigernán Mór Ua Ruairc, who reigned from 1124 to 1172, was conquering eastward and it was under Tighernán Mór that the Kingdom of Breifne reached its greatest expanse, extending from Hill of Ward and Kells, County Meath to Drumcliff, County Sligo in the late 12th century. Tighernán Mór consolidated the eastern territories he had conquered through his marriage to Derbforgaill, daughter of the King of Meath.
There was great animosity between the subjugated O'Reillys and their O'Rourke overlords. Tighernán Mór went to war with King Diarmait Mac Murchada of Leinster in 1152 following Mac Murchada's abduction of Queen Derbforgaill, and his claim over land that was in Brefnian possession. Taking advantage of Breifne's preoccupation, chief Geofraidh O'Reilly launched an unsuccessful rebellion in 1154 and was banished from the kingdom following his defeat. In 1155 when Donnchad Ua Cerbaill king of Airgíalla was captured and imprisoned by Tighernán Mór, Geofraidh O'Reilly and his supporters ambushed the Brefnian guards and rescued Ua Cerbaill. O'Reilly was later captured and executed for this act of sedition.
Norman invasion
Tighernán Mór formed a coalition with the High King, Ruaidrí Ua Conchobair, which ousted Mac Murchada in 1166. Mac Murchada fled to England and sought help from Henry II to aid him in reconquering his kingdom. This prompted the Norman invasion of Ireland in 1169. With Norman aid Mac Murchada retook Leinster, and set Kells, Breifne's easternmost outpost, ablaze. In 1170 Breifne was invaded and in the following years the kingdom was carved up by the Normans. O'Rourke power was weakened whilst the O'Reilly submitted to Mac Murchada and his Norman allies during the conquest.
Shortly after Mac Murchada's death in May 1171, most of the native Irish kingdoms waged war on his successor, Richard de Clare or "Strongbow". However, the O'Reilly allied themselves with the Normans as a way of breaking free of the O'Rourke. The division between the two clans is most apparent during the Siege of Dublin, when the O'Rourke king is encamped outside the city with his Gaelic allies, and the O'Reilly king is inside aligned with Strongbow and his council.
During parley in 1172 at Trim, Tighernán Mór was betrayed and killed by Hugh de Lacy, Lord of Meath, throwing the O'Rourke dynasty into chaos. The instability and wars of succession between the various branches of the O'Rourke sept weakened their hold over the territory of Breifne even further. With the help of de Lacy, the O'Rourkes were driven back and expelled from O'Reilly land. Good relations between the O'Reilly and the Normans persisted until the early 13th century when they were soured by Hugh's two sons, Walter and William Gorm.