The families of this name are another family to have often resumed the use of the prefix 'O'. They are still found mainly in Munster, where the O'Riordan sept was located in county Tipperary. The townland of Ballyreardon in Barrymore barony, county Cork commemorates the home of a branch of the sept which established itself there. The maritime county of Cork, in Munster, is bounded by the sea on the south-west, the south and the south-east. To the east it has land boundaries with the counties of Waterford and Tipperary, and to the north with Limerick and to the west with Kerry. Anciently the country formed part of the kingdom of Desmond. After the Anglo-Norman Invasion the whole of the present county, save the City of Cork (which had been founded by the Vikings) and its surroundings, was granted in 1177 by Henry 11 to Anglo-Norman knights who brought over their followers and established a military colony. Ireland was one of the earliest countries to evolve a system of hereditary surnames. They came into being fairly generally in the 11th century, although a few were formed before the year 1000. When the sparse Irish population began to increase it became necessary to broaden the base of personal identification by moving from single names to a more definite nomenclature. The prefix MAC was given to the father's christian name, or O to that of a grandfather or even earlier ancestor.
Most bearers of this name claim descent from an eponymous ancestor, killed at Aherlow in 1058, who was the son of Cucoirne, Lord of Ely O'Carroll.
The associated arms are recorded in Sir Bernard Burkes General Armory. Ulster King of Arms in 1884. They were a sept of some note in Muskerry and County Cork, and were distinguished as military chiefs. The name in Irish is RIOGH BHARD, meaning 'Royal bard'.
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