O’Reilly, ó Raghallaigh A Gaelic surname meaning descendant of Raghallach, which personal name was from the Old Norse and introduced by the Vikings. The principal sept of Brifne (Breffny), who are of the same stock as the ó Ruairc (O’Rourke). They were Chiefs of Breffny-O’Reilly, which originally comprised the greater part of the present Cavan. In 1890 O’Reilly was the eleventh most numerous surname in Ireland, the estimated number of bearers was 29,000, and it was the most numerous surname in Cavan, Longford and Meath. In that year the surname was found in every county in Ireland. It was principally found in Cavan, Longford, Dublin, Meath, Mayo and Cork. In the United States Reilly is the 901st most numerous surname with an estimated 35,750 bearers, and O’Reilly is the 3,584th most numerous surname with an estimated 8,250 bearers. It is numerous in the states of Connecticut, New Jersey, New York, and Pennsylvania. In England and Wales in 1996 Riley was the 153rd most numerous surname. In Scotland in 1995 Reilly was the 98th most numerous surname.
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