O'Brien (Ó Briain), Brian Ruad (d. 1277), son of Conchobar O'Brien (qv), king of Thomond, ruled Thomond during an intensely turbulent period of its history. He succeeded after his father's death at the hands of his kinsmen in 1268. At first he continued his father's policy by ravaging the colonists north of the Shannon, taking the castle of Clare in 1270. However, his position was compromised and weakened by the emergence of a powerful rival, his nephew Toirrdelbach O'Brien (qv). Despite enjoying early success over his nephew, Brian Ruad steadily began to lose ground, and in 1273 he was forced to submit to the justiciar. By 1275 the O'Brien civil war had reached stalemate, with neither protagonist being able to defeat the other decisively. However, this delicate balance shifted when Edward I granted Thomond to Thomas de Clare (qv). With royal assent, de Clare was to subdue the O'Briens and bring Thomond back into the land of peace.
Brian Ruad attempted to use de Clare to outmanoeuvre his rival. To this end he formed an alliance with de Clare, becoming his ‘gossip’ or ‘brother in Christ’. Now with de Clare and FitzGerald backing, Brian sought out his young rival for a telling encounter. However, Toirrdelbach had other ideas and secured help from the de Burghs of Connacht; and on the battlefield Toirrdelbach triumphed over his uncle in 1277 . Brian Ruad fled the field to the refuge offered by de Clare's castle of Bunratty. Once inside the walls, he was taken and condemned to death by his ally; the luckless Brian was then shackled to four horses and torn asunder. Why de Clare turned on him remains a mystery, but obviously the colonist must have deemed that O'Brien's reverse was irrecoverable and that he was of little further use. The sons of Brian Ruad, led by the eldest, Donnchad (c.1260–84), retaliated against de Clare, although political practicalities forced them to renew the alliance with him in 1278. But Toirrdelbach now exercised the upper hand over his cousins. In 1281 Donnchad and his brothers came to terms with him through the separate efforts of Domnall Ruad MacCarthy (qv) of Desmond and the government. Thomond was partitioned, and both leaders accepted de Clare overlordship. This unsteady peace proved ephemeral, and soon warfare exploded among the cousins, culminating when Toirrdelbach killed Donnchad at a parley (1284). This put a stop to the conflict, but the tension continued to simmer and boil beneath the surface of an uneasy truce.
When Toirrdelbach died (1306) he left his son and successor Donnchad O'Brien (qv) to reap the whirlwind. The succession sparked another O'Brien civil war, with Brian Ruad's grandson Diarmait Cléirech (c.1280–1313) providing the opposition. With de Clare backing, Diarmait Cléirech defeated Donnchad and his de Burgh allies near Bunratty (20 May 1311), and Donnchad's killing in Corcomroe shortly afterwards secured the victor's elevation to the Thomond kingship. However, Diarmait Cléirech faced continual and dogged resistance from his rivals till his own peaceful death in 1313.