From Ó Raghallaigh, rulers of Breifne — the old kingdom that gave modern County Cavan its shape.
Gaelic Origin · Root name Raghallach
O'Reilly descends from the personal name Raghallach, of disputed but likely martial meaning. The Ó Raghallaigh ruled East Breifne — essentially modern Cavan — as one of Gaelic Ireland's most enduring lordships, minting their own coinage in the fifteenth century.
Few Gaelic families held their own ground so long, or so completely, that the land itself still bears their memory.
Irish form: Ó Raghallaigh
