Brian

The O'Briens, Irish Ó Briain, descend from Brian Boru of the Dál gCais, the Munster king who broke the Uí Néill hold on the high kingship, ruled all Ireland from 1002 and fell winning Clontarf against Dublin's Norsemen and their Leinster allies in 1014. His kin took the name Ua Briain, supplied two more high kings and ruled Munster until the 1118 Treaty of Glanmire split the province and left them Thomond.

The Norman invasion drove them beyond the Shannon, and in 1276 the Crown granted Thomond to Thomas de Clare; the de Clare bid ended in defeat at Dysert O'Dea in 1318, Thomond staying independent for two centuries. Succession ran by tanistry, electing the ablest royal kinsman, which fed long internal feuds.

Last king Murrough O'Brien surrendered to Henry VIII and in 1543 was made Earl of Thomond. A later branch, the Viscounts Clare, followed James II into exile and was attainted in 1691; the earldom was forfeited in 1741.