O'Brien

The Ó Briain dynasty descended from Brian Boru ruled the kingdom of Thomond — north Munster, principally modern Clare with much of Limerick and Tipperary — for six centuries after his death at Clontarf. The family seat was at Kincora on the Shannon at Killaloe; the inauguration site at Magh Adhair near Quin in Clare was the political-ritual centre. After the Anglo-Norman conquest the O'Briens negotiated; they kept Thomond, retitled themselves earls of Thomond and barons of Inchiquin under English peerage from 1543, and survived as a great Anglo-Irish house into the 19th century with a continuous male-line descent from Brian Boru that is among the longest documented in Europe.

Murchadh O'Brien, 6th Baron Inchiquin (c.1614–1674) — known to history as 'Murrough of the Burnings' for the methods of his Confederate-then-Royalist career — was the most controversial bearer of the name in the 17th century. William Smith O'Brien (1803–1864) led the Young Ireland rising of 1848 and was transported to Van Diemen's Land. Conor Cruise O'Brien (1917–2008), the diplomat, historian and politician, was the most consequential Irish public intellectual of the late 20th century.

The senior line — Donough O'Brien, 18th Baron Inchiquin (b. 1943) — remains the only Gaelic dynastic head whose chiefly title is recognised in continuous unbroken male-line descent from a pre-Norman Irish king. The line predates the Plantagenet, Capetian and Habsburg royal houses by more than a century.