Ua Cerbaill, Donnchad (d. 1168/9), king of Airgialla (Oriel) and noted church reformer, was son of Cú Chaisil of the Uí Duibne branch of the Fernmag dynasty from which the name of the barony of Farney, Co. Monaghan derives. His mother was Aillend, daughter of Ua Baegelláin, a local ruler of Fir Manach; she was also mother of Tigernán Ua Ruairc (qv), who was thus Donnchad's half-brother. The first recorded act of Donnchad's reign is his conquest (c.1130) of the kingdom of Conaille Muirthemne in north Louth.
In 1147 Muirchertach Mac Lochlainn (qv), king of Ailech, allied himself with Donnchad in defeating the Ulaid of east Ulster, taking rich spoils and a number of valuable hostages. Muirchertach subsequently assumed the high-kingship of Ireland; having conquered Mide, he divided up the greater part of it with his allies Ruaidrí Ua Conchobair (qv) of Connacht, Tigernán Ua Ruairc, and Donnchad. Ua Ruairc and Ua Cerbaill later demonstrated their jurisdiction over Mide by giving grants of land for new church foundations within it. In 1150 Donnchad took over the Brega sub-kingdom of Fir Arda Cianachta; in alliance with Tigernán he also took over most of the territory of Mide. Although Donnchad claimed the kingship of all Airgialla, he was opposed to some extent by certain minor dynasties within it, such as the Airthir of Armagh. Nevertheless, he was able to install his son Murchad Ua Cerbaill (qv) as king of the territories of Uí Méith and Airthir.
Donnchad, alone among the kings of Ulster, clearly supported the movement for church reform promoted by Cellach (qv) (d. 1129), Malachy (qv) (d. 1148), Áed Ua Cáellaide (qv) and others. In collaboration with Malachy, he facilitated the establishment of the first Irish Cistercian foundation by providing lands at Mellifont (Co. Louth) in 1142. His support was further manifested through his patronage of St Mary's abbey at Louth, which was founded that same year, St Mary's being the Irish mother house of the Augustinian canons regular of Arrouaise – as Mellifont was of the Cistercians. All Hallows priory, Dublin, which was established through the influence of Archbishop Lorcán Ua Tuathail (qv), became a daughter house of St Mary's.
At the request of Donnchad, Malachy granted all the territory of Louth that lay within the archdiocese of Armagh to the diocese of Clogher, and installed Áed Ua Cáellaide, bishop of Clogher–Louth as the first abbot of St Mary's. Áed transferred his cathedral church from Clogher to Louth, so that in effect the diocese of Clogher became the diocese of Louth. It appears, however, that Gilla Meic Liac (qv) the comarba (successor) of St Patrick (qv) opposed the removal from Armagh of territory allocated to it at the synod of Ráith Bressail (1111). The Annals of Tigernach (Tigernach Ua Bráein (qv)) for 1152 record: ‘Gilla Meic Liac was outraged and wounded by Ua Cerbaill, king of Airgialla; and because of that Ua Cerbaill was plundered and deposed by the son of Mac Lochlainn’, namely Muirchertach. While Donnchad was temporarily deposed, he subsequently recovered his kingdom and reigned for another fifteen years. With Donnchad's collaboration, Áed later established another Arrouaisian house, at the ancient monastic site of St Féchín (qv) at Termonfeckin, near Drogheda. Donnchad thus had a major role in the introduction into Ireland of both the Cistercians and the Augustinians, the two principal orders of the reform movement.
In 1165 Eochaid Mac Duinn Shlébe ceded the Ulster sub-kingdom of Bairrche to Donnchad. In that year Muirchertach Mac Lochlainn ravaged the lands of the Ulaid and expelled Eochaid, who was held captive by Donnchad. Eochaid was released under specific terms of surrender: Muirchertach promised to restore him as a tributary king, swearing on the Bachall Ísu – an important Patrician relic – in the presence of Archbishop Gilla Meic Liac, Donnchad and others. At Easter 1166, however, Muirchertach recaptured Eochaid and had him blinded in contravention of his previous guarantee. Donnchad, together with the forces of Bréifne and Conmaicne, then entered Muirchertach's territory and defeated and killed him in battle at Leitir Luin, in the Fews of Armagh. Donnchad had previously entered into an alliance with Ruaidrí Ua Conchobair and surrendered hostages to him at Drogheda. He may also have played a part in the banishment from Leinster of Diarmait Mac Murchada (qv), later known as ‘king of Leinster and the foreigners’.
In addition to Murchad who succeeded him, Domnall also possibly had a second son, Niall (fl. 1196–1208), who founded the Mac Mathgamna dynasty of Airgialla. In 1168 Donnchad was gravely wounded by a servant of his own people in a drunken brawl; he apparently survived long enough to have ‘the victory of unction and penance’ and to bestow 300 ounces of gold on the church. His obit in the Antiphonary of Armagh (1 January 1170) testifies fulsomely to his patronage and to his endeavours on behalf of the reformist party in the Irish church: ‘A prayer for Donnchad Ua Cerbaill, supreme king of Airgialla, by whom were made the book of Cnoc na nApstal at Louth and the chief books of the order of the year, and the chief books of the mass . . . By him the church throughout the land of Airgialla was reformed and a regular bishopric was made, and the church was placed under the jurisdiction of the bishop . . . .’