The Clan/Sept History

O'Reilly is the most common surname in Cavan. The O'Reillys trace their ancestry back to Conn Ced-catchach ( Conn of the hundred battles). One of his descendants was Brian, King of Connaught, and in the 4th century, his descendants became known as Ui Briuin (the race of Brian). Brian had 24 sons, and from one came a son called Dui Galach who was ancestor of the O'Connors, the O'Flahertys, the O'Rourkes and the O'Reillys. He had a son Fergus, whose own son Feargna, was ancestor to Ui Briuin Brefnie (the O'Reillys and the O'Rourkes).
Sometime round the 6th century Feargna migrated north, and was succeeded by a son Aedh Finn (Hugh the fair) who died in 611 A.D. Aedhs son Maelmorda was ancestor of the O'Reillys. This is why for much of the time they ruled what is now county Cavan, it was known as Muinter Maelmordha (The country of Maelmordha's people). From Maelmordha through Dubhcren and Cathalan came Raghallach, who was reputedly killed at the Battle of Clontarf. From him all the O'Reillys are named. Such is the ancient genealogy of the O'Reillys.