The O'Reillys were historically the kings of East Bréifne in what is today County Cavan.

O'Reilly is ranked tenth in the top twenty list of most common Irish surnames.

[citation needed] Usually anglicised as Reilly, O'Reilly or Riley, the original form of the name, Ó Raghallaigh, denotes "descendant of Raghallach".

The Ó Raghallaigh family were part of the Connachta, with the eponymous Raghallach said to have died at the Battle of Clontarf in 1014.

[5] A self-proclaimed and disputed O'Reilly Clan Chieftain to this day is at odds with the O'Rourke Clan Chieftain because he contests the recognised O'Rourke claim on the title Prince of Breifne.