O'Brien

Successful Gaelic Irish dynasty claiming descent from the great chieftain Brian Bóruma, who became high king of Ireland in 999. The O'Briens established a key strategic position in Thomond, County Clare, in the 13th century through resistance to and negotiation with the Anglo-Norman invaders. Their lordship was divided into the houses of Thomond and Inchiquin in the 1570s, but these were joined after 1774 when the earldom of Thomond died out.

Having successfully repulsed several challenges from the Anglo-Normans, notably with their victory over the Anglo-Normans at the Battle of Dysert O'Dea in 1318, the O'Briens entered into a surrender and regrant treaty with the crown in 1543, under which they surrendered their lands and received them back with confirmation of their title. However, highly complicated succession disputes led to the division of their lordship into the house of Thomond and the house of Inchiquin. The division proved highly stabilizing, and the O'Briens survived the turmoils of the 17th century maintaining a largely Royalist stance throughout. Though some of the ruling family remained loyal to the Catholic Jacobite supporters of James II, the majority supported the cause of the Protestant king William (III) of Orange, and prospered in the 18th century. Although the earldom of Thomond became extinct in 1774, the Inchiquin line, amalgamated with Thomond, survives to the present.