Mac Gilla Mo Cholmóc, Domnall (d. 1185), king of Uí Briúin Chualann, is of obscure origins, except that the branch of the Mac Gilla Mo Cholmóc dynasty to which he belonged was closely allied to Diarmait Mac Murchada (qv) (d. 1171), king of Leinster, to whose daughter, Derbforgaill, Domnall was married. In 1166 a crisis engulfed Mac Murchada when his ally the high-king Muirchertach Mac Lochlainn (qv) (d. 1166) blinded Eochaid Mac Duinn Sléibe, foster-son of Donnchad Ua Cerbaill (qv) of Oriel. O'Carroll transferred his allegiance to the king of Connacht, Ruaidrí Ua Conchobair (qv) (d. 1198), who then moved against Mac Lochlainn and Mac Murchada by marching to Dublin to be crowned high-king; it appears that Domnall's kinsman, the reigning king of Uí Briúin Chualann, also switched his loyalty from Mac Murchada to Ua Conchobair, but Domnall himself made no move. Ruaidrí took the submissions of the Uí Dúnlainge princes of west Leinster and attacked Mac Murchada in his heartland of Uí Chennselaig, forcing him to submit. He then turned north and marched to Donegal, leaving Mac Murchada a breathing-space to reassert his authority over Leinster. Mac Murchada used this opportunity to take decisive action against the Meic Gillamocholmóc by encouraging the Uí Braenáin to kill the rebel king of Uí Briúin Chualann. The king's death allowed Domnall to claimed the kingship, but also sealed Mac Murchada's fate as it sparked a second revolt by the Uí Dúnlainge princes of west Leinster, which ultimately unseated Mac Murchada and forced him to flee for help to Henry II (qv) of England. Although Domnall was Mac Murchada's son-in-law he made terms with the west Leinster princes.
In 1167 Mac Murchada returned from England with a large contingent of English troops and reconquered Uí Chennselaig; Domnall seems to have joined Mac Murchada about 1169. With the help of his other son-in-law, Richard fitz Gilbert de Clare (qv) (d. 1176), earl of Pembroke and Striguil, Mac Murchada now challenged Ruaidrí Ua Conchobair for the high-kingship of Ireland, a bid that ended abruptly with his death at Ferns in May 1171. After Mac Murchada's death the Leinster nobility saw their chance to confront Clare and his ally Mac Murchada's son Domnall Cáemánach Mac Murchada (qv) (d. 1175), now king of Leinster. Domnall's allegiance to the Mac Murchada faction was swayed by the new political climate and in summer 1171, when the deposed Ostman king of Dublin, Asgall (qv) (d. 1171), attempted to retake his city, he changed sides. Miles de Cogan (qv) (d. 1182), who held hostages of Domnall, approached him to establish his intentions in the struggle over Dublin, and Domnall confirmed his loyalty to the English. Cogan returned the hostages to Domnall, but made a strange proposal – that the king should remain neutral until the tide of battle changed in favour of one side or the other and should then join the winning side. Domnall agreed, and once the struggle turned against Asgall and his Ostmen joined the English in their rout of the fleeing Dubliners.
About a month later Domnall deserted the English and joined the advancing Leinster army led by Murchad Mac Murchada (qv) (d. 1172), king of Uí Chennselaig, and Archbishop Lorcán Ua Tuathail (qv) (d. 1180). They positioned themselves at Dalkey as part of the blockade of Dublin mounted by the high-king, Ruaidrí Ua Conchobair. But after the English broke the siege in autumn 1171, Domnall again turned against the Irish and accepted Clare's overlordship. His survival was ensured by his being related to the MacMurroughs (Meic Murchada): he was brother-in-law to both Clare and Domnall Cáemánach Mac Murchada, and two of his sons were Mac Murchada's grandsons. In winter 1171–2 Domnall was one of the Irish princes who submitted to Henry II, and by the terms of his submission was granted tenure of his lands directly of the king, which guaranteed his dynasty's survival as major landowners for a further century and a half; however, he lost his lands at Santry, Raheny, and Clontarf in north Dublin, which were granted to English newcomers.
Domnall and his dynasty quickly assimilated to the new order, as is evident from his witnessing of Clare's charter confirming Glendalough's church lands in 1172, and he joined Clare's campaign into Meath during 1173. A document from the reign of Archbishop Luke (qv) of Dublin (d. 1255) reveals more about Domnall's commitment to the new regime: Luke states that during the reign of Archbishop John Cumin (qv) of Dublin (d. 1212) ‘Gilleholmoc and other good men’ enclosed a common of turbary and pasture on the mountain called Slestoll. A shrewd politician and an astute champion of his people's interests, Domnall died about 1185. He was succeeded by his son, Diarmait Mac Gilla Mo Cholmóc (qv) (d. c.1216).