O'Dwyer (Irish: Ó Dubhuir), also known as Dwyer, Dyer is one of Ireland's oldest Gaelic noble or aristocratic houses, based most prominently in what is today County Tipperary.

[4] A claimed ancestor of the O'Dwyer clan was King Milesius of Spain, who according to legend conquered Ireland for the Gaels.

[10] The O'Dwyers eventually emerged as Lords of Kilnamanagh in the High to Late Middle Ages, but they are not referenced by name as significant in the Annals of the Four Masters until the 15th century.

[11] Along with the O'Carroll of Éile, the O'Kennedy of Ormond and the Mulryan of Owney, the O'Dwyers of Kilnamanagh were one of a cluster of regional Gaelic clan powers in the High Middle Ages in the area of what would one day become County Tipperary who held out against Anglicisation with the arrival of the Lordship of Ireland.

They interplayed with newer Norman arrivals on their borders who became significant powers, especially the Butler Earls of Ormond.