In 1237 Cathal O Reilly, Prince of Breifne, founded the monastery of Lough Oughter. The Franciscan abbey there was founded in 1300 by Giolla Iosa Ruadh O Reilly and it contains the tombs of several O Reillys. From AD 243 to 1523, there were 39 O Reilly abbots at Kells, County Meath. Saint Oliver Plunket, canonized in 1975, is among their family connections. Since the sixteenth century, five O Reilly bishops have held the primacy of Armagh, as well as five other bishoprics.358

Opposed William "Gorm" de Lacy in his invasion of Breffny and defeated him in 1233, at which battle many were slain. He founded a monastery on Trinity Island on Lough Uchter in 1244, and was addressed as "King of Breffny by King Henry III in that king's plea for aid against the Scots. He was killed at the battle of Moysleaghta in 1256.9