Donnchad Ua Cerbaill or Donnchadh Ó Cearbhaill, king ofAirgíalla, fl. 1130s–1168. Ua Cerbaill was a supporter of the Irish religious reform movement of the 11th century. He was a close associate of Saint Malachy, and with him founded Mellifont Abbey, the first Cistercianabbey to be built in Ireland. He was also responsible for foundations by the Arrouaisian order at Louth, Knock Abbey and Termonfeckin. He was central to the successful overthrow of Muirchertach Mac Lochlainn, High King of Ireland, who in violation of an oath had blinded Donnchad's foster-son Eochaid Mac Duinn Sléibe the king of Ulaid in 1166. Afterwards he placed Áed in Macáem Tóinlesc on the throne of Tír Eogain. He was a half-brother of Tigernán Ua Ruairc, King of Bréifne. Ua Cerbhaill was a successful political operator on both a regional and local level. His religious foundations show his success in tying the reform movements interests to his own, facilitating as they did the consolidation of conquests he made. In alienating religious sites from local dynasts he undermined their use as a power base for future opposition and created a vested interest for the religious orders in maintaining the status quo. He was able to incorporate the Ulaid sub-kingdom of Conaille Muirtheimne, and the Midhe sub-kingdom of Fir Arda Ciannachta into his kingdom, the basis of modernCounty Louth, during the "High Kings with Opposition" period of instability which saw provincial armies repeatedly criss-cross the area, multiple divisions of Midhe, and frequent changes of kings. To reflect this change, between c. 1140 to c. 1190, County Louth was transferred from the see of Armagh to the see of Clogher. During this period the Bishop of Clogher used the style Bishop of Louth. This change reflects the shift in center of the Ua Cerbhaill Uí Chremthainn sub-kingdom south within Airgíalla from the time of the Synod of Ráth Breasail in 1111 to the 1130s when Ua Cerbaill was establishing his base of power in Fernmag and around the settlement of Louth. This shift was precipitated by expansion of the Cenél Fearadhaigh Theas, a powerful Cenél nEoghain group, into the Clogher Valley. The title Bishop of Clogher was resumed after 1193, when County Louth was restored to the see of Armagh. By that time the Ua Cerbhaill family had been eclipsed and County Louth or 'English' Oriel had been lost to the Norman colony.
