O'Brien (Ó Briain), Donnchad Cairprech (d. 1242), king of Thomond, was one of nine sons of Domnall Mór Ua Briain (qv), king of Thomond, and Orlaith, daughter of Diarmait MacMurrough (Mac Murchada) (qv). In his youth, Donnchad Cairprech was fostered in the Limerick territory of Uí Cairbre Aobha. After Domnall's death in 1194, his dynasty was engulfed by the struggle for the kingship. Initially Donnchad Cairprech and his two most important brothers, Muirchertach Finn (d. 1239×1241) and Conchobar Ruad (d. 1203), stood together and picked off the challenges of their cousins one by one. Having won the war, they were bitterly divided on how to implement the peace. It soon became clear that Muirchertach Finn had the support of his dynasty and was elected king. At first Donnchad Cairprech refused to accept this decision and installed himself as king with colonist aid. The division between the three brothers was so intense that they could not decide who should possess Limerick town. In the end they abandoned it, allowing the colonists to return. In time, relations settled down between Donnchad and Muirchertach, with the former recognising the latter's kingship. Despite limited O'Brien participation in the attack on the colony in Munster by their brother-in-law Cathal Crobderg Ua Conchobhair (qv) (O' Connor) there is evidence of good relations between the colonists and the O'Briens at Limerick. Indeed, a sister of these brothers was married to the powerful William de Burgh (qv). However, the O'Briens were intent on using the colonists against their old enemies the MacCarthys (Meic Carthaig) of Desmond, who burnt Limerick in 1196. Their alliance with de Burgh provided the O'Briens with extra muscle against MacCarthy, forcing the latter to come to peace in 1199. But the feud between the brothers erupted again that year. Then Conchobar Ruad and the colonists first attacked Donnchad Cairprech before besting and exiling Muirchertach Finn in 1198. After this success Conchobar Ruad assumed the kingship, while Muirchertach Finn was imprisoned in a colonist gaol in 1199. Despite turbulence the brothers still enjoyed generally good relations, as evidenced by their probable approval of the grants of King John (qv) on both sides of the Shannon in 1199. And later (1201) the property of the see of Limerick was ratified by an inquisition presided over by de Burgh and including Conchobar Ruad as a juror.
The O'Briens also used the colonists as a stick to beat their inveterate O'Connor rivals by supporting de Burgh's offensives into Connacht. They also gave their covert support to the intrigues of the O'Flahertys (Uí Fhlaithbertaig) against Cathal Crobderg, and Donnchad Cairprech seemingly became ‘gossip’ (a bond formed by standing joint sponsor at a baptism) with Áed O'Flaherty about this time. Indeed, Muirchertach Finn's decision to join de Burgh earned him his release, campaigning three times in 1200–02. However, Muirchertach had his revenge on Conchobar Ruad, killing him to retake the kingship in 1203 . In 1208–9 Donnchad Cairprech made his bid for power in Thomond and deposed Muirchertach Finn with the help of the colonists of Limerick. However, he allowed Muirchertach to retain some of his power, as an English roll mentions a ‘Mariadac’ as king of Limerick in 1210. Also that year Donnchad Cairprech and Muirchertach Finn joined Cathal Crobderg and Áed O'Neill (qv) to campaign for King John against the de Lacys. The bond between the colonists and the O'Briens was further ratified at Waterford in 1210, when Donnchad Cairprech was knighted after making his submission. In addition he was granted the Limerick lordship of Carrigogunnell for an annual rent. He further confirmed his loyalty to John by campaigning against Cathal Crobderg in 1210, and by his presence at the defeat by Cormac O'Melaghlin (qv) (Ó Máel Sechlainn) of the army of Justiciar John de Grey (qv) at Kilnagcrann, Co. Westmeath (1212). Two years later this active warrior king was back in the saddle, fighting the MacCarthys. Thereafter a long period of peace followed. The next reference to him comes in the summer 1224. Then Donnchad Cairprech, Áed O'Connor (qv), and Diarmait Cluasach MacCarthy campaigned with William Marshal II (qv) against the de Lacys. On his return to Thomond, he and the colonists answered Áed O'Connor's call for assistance by driving the sons of Ruaidrí Ua Conchobair (qv) (O'Connor) and Áed O'Neill out of Connacht. In 1225 he was back fighting in Connacht for Áed, while Muirchertach Finn joined a rival invasion of Connacht by the colonists of Munster and Leinster.
However, grants to the colonists gradually pushed the O'Briens into Clare. There Donnchad Cairprech built a new capital at Clonroad near Ennis, holding his lands of the king for an annual rent. In the years before Donnchad's death, there was growing anger among the O'Briens because of these grants. And in 1235 a disillusioned Donnchad Cairprech supported Fedlimid O'Connor's (qv) bid to defend his kingdom from Richard de Burgh (qv). However, a mistake by Donnchad Cairprech cost them the decisive battle, convincing him to come to peace. After the death of Muirchertach Finn (1239×1241) and his own in 1242, a new generation of O'Brien leaders emerged. Donnchad Cairprech married Sadhb, daughter of Donnchad O'Kennedy (Ó Cennéidig), lord of Muscry Tire; they had two daughters and six sons. Apart from his military achievements, Donnchad Cairprech was also a noted founder of abbeys, sponsoring the abbey of Corcomroe in the Burren, the Franciscan abbey at Ennis, and Killcooley in Tipperary. He was succeeded by his son, Conchobar O'Brien (qv), as king of Thomond in 1242 .