Following the death of his granduncle Domnall mac Amalgada in August 1105, Cellach succeeded as abbot of Armagh and Coarb Pátraic. The
Annals of Ulster notes that this was done "by the choice of the men of Ireland". Unlike his lay predecessors/ancestors, he sought priestly ordination, which Flanagan has described as a "decisive reform step". The incumbent bishop of Armagh, Cáenchomrac Ó Baígill, died in 1106. This offered the opportunity for Cellach to unite the abbatial and episcopal office, and when he the same year visited
Munster he "assumed the orders of a noble bishop by the command of the men of Ireland". That this was done in Munster may be an indication of the influence held by
Muirchertach Ua Briain (Murrough O'Brian) Gilla received a letter from Anselm congratulating him on his elevation to the see of Limerick, and there was no suggestion that Anselm felt Canterbury had been slighted or ought to have been involved. The records of the visitations are followed by formulas such as "and he obtained a full tribute" or "brought away his full dues", indicating the official character of ecclesiastical submission embedded in such visitation. Cellach also conducted negotiations between rivalling secular rulers, in particular between Muirchertach Ua Briain and the Northern Uí Néill claimant for high-kingship,
Domnall Ua Lochlainn of the Cenel nEógain. There are records of Cellach making "a year's peace" between these two in the entries of the Annals of Ulster for 1107, 1109 and 1113. During his incumbency the priory of Sts. Peter and Paul at Armagh was re-founded by Imar, the learned preceptor of St. Malachy. This was the first establishment in Ireland into which the Canons Regular of St. Augustine had been introduced. ==Synod of Ráith Bressail and Dublin==