Riordan, with its variants O'Riordan and Reardan, comes from the Irish original O Rioghbhardain (O Riordain in modern Irish), riogh- meaning "royal", and bardan a diminutive of bard, "poet". The surname originated in the area between the modern towns of Thurles in Co. Tipperary and Birr in Co. Offaly known as "Ely O?Carroll", the kingdom of Eile. With the rise of the O?Briens in the tenth and eleventh centuries, this kingdom was fragmented and by the twelfth century, the O Rioghbhardain are recorded further south, in Co. Cork. Their migration may have taken them through east Cork, where the townland of Ballyreardon bears their name but by the 15th and 16th centuries they were securely settled in the west of the county, in Muskerry particularly, where they were faithful followers of the MacCarthys. With the MacCarthys, they were active in the rebellions of the late sixteenth century and at the battle of Kinsale in 1602; after that defeat many sailed away with the Spaniards and numerous O?Riordans are recorded in the Spanish army in Flanders in the seventeenth century. The family were also active in the Jacobite cause later in the century. Daniel O?Riordan, a captain in Dillon?s Regiment in the French army, obtained admission to the French nobility in 1700 and the family are still well known in the areas around Nantes.
The strength of their association with their homeland remains very strong; a large majority of those bearing the name originate in Co. Cork.
The resumption of the "O" prefix by the family has been remarkable: whereas 94% of births of the name in 1890 are for "Riordan", by 1996 almost 70% of households are "O?Riordan".
Sean P. O Riordain (1905-1957), born in Monkstown Co. Cork, was one of the most important archaeologists of his generation, becoming Professor of Archaeology in University College Cork in 1936, and Professor of Celtic Archaeology in University College Dublin in 1943.
Sean O Riordain (1916-1971). Born in Ballyvourney, Co. Cork, O Riordain spent most of his life in Cork city as an official of Cork Corporation. Even before the publication of his first book of poems "Eireaball Spideoige" in 1952 his unique talent was widely recognised and he is now considered by many to have been the finest Irish-language poet of the twentieth century.