The name O'Reilly comes from the Irish chieftain Ragheallach, who lived at the time of Brian Boru and, like him, was killed at the Battle of Clontarf in 1014. He was a great-grandson of Maomordha, a descendant of the O'Conors, kings of Connacht. Today, it is one of the most numerous names in Ireland.

Co. Cavan is a particular stronghold of the name. Myles "The Slasher" O'Reilly was the heroic defender of the bridge at Finea in Co. Cavan in 1646 where he and a force of one hundred held out against a 1,000-strong Cromwellian army. O'Reilly is commemorated by a cross in the main street of Finea, a pretty village on the banks of the River Inny.

The most famous holder of the name today is Dr. A. J. F. O'Reilly, the wealthiest man in Ireland. He is the head of the Heinz Corporation and Independent Newspapers, Dublin, and used to play rugby for Ireland.