Gilla meic Liac (Gelasius) (d. 1174), abbot of Derry, archbishop of Armagh, was the son of Annaid, grandson of Ruaidrí, and belonged to a line of the Cenél nÉogain (RIA, MS 23. D. 17). The description of his father as in fer dána (the brave man) suggests that the latter had pursued a secular rather than an ecclesiastical career. In 1121 Gilla meic Liac, already approaching his mid-thirties, became airchinnech (superior) of the Columban foundation of Derry, a position which he held for sixteen years. Under his administration Derry was increasingly recognised as a centre of monastic fervour. Gilla meic Liac's commitment to reform was so well understood that Máel-Máedóc (St Malachy (qv)) was clearly satisfied to choose him as his successor when circumstances forced his resignation from Armagh in 1137. Following his election to the see, Gilla meic Liac, already in priest's orders, was raised to episcopal rank. As bishop of Armagh, Gilla meic Liac was in effect provincial: the synod of Ráith Bressail in 1111 had left Armagh at the head of the more northerly of two ecclesiastical provinces. It was not until 1152, however, at the synod of Kells–Mellifont, that he was accorded the archiepiscopal prerogative of a pallium and was formally acknowledged as primate. His status as a reformer is reflected in the working relationship that he clearly had with Malachy, in the latter's capacity as papal legate. It can be inferred that Malachy intended to obtain a pallium for Gilla meic Liac before his own untimely death (1148).
As archbishop of Armagh, Gilla meic Liac became a major contributor to the church reform of the twelfth century. He worked closely with the new papal legate, Gilla Críst (Christian) Ua Conairche (qv) and in 1158 the two convened a synod at Brí Meic Taidc (perhaps Braymount, parish of Laracor, Co. Meath). Three years later, Gilla meic Liac called a synod at Clane (Co. Kildare), which sought to promote uniformity of doctrine throughout Ireland with a decree that all lectors should be alumni of Armagh. By the time the second synod of Cashel took place in 1172 Gilla meic Liac, who was by then an octogenarian, was too ill to attend. Nonetheless, he recovered sufficiently to travel to Dublin and subscribe to the decrees of the synod, which sought to bring the Irish church into conformity with that of England and to make the payment of tithes compulsory. Aside from the pursuit of reform through synodal decree, he was active in promoting the new continental religious orders in Ireland. He was presumably responsible for the introduction to Armagh of the Augustinian canonesses, a development which apparently took place in the 1140s. He approved the foundation of the Cistercian house at Newry in 1157 by the claimant to the high-kingship of Ireland, Muirchertach Mac Lochlainn (qv). A year later he was party (with the papal legate Christian) to the consecration of the Cistercian abbey at Drogheda. Gilla meic Liac was clearly concerned with maintaining the authority of Armagh, not only within its own ecclesiastical province but within Ireland as a whole. In his capacity as comarba Pátraic (successor of St Patrick (qv)), he made a visitation of Munster in 1138, of Connacht in 1140 and again 1151, and of Cenél nÉogain in 1150. At the synod of Clane in 1161, he formally reaffirmed the primacy of Armagh within the Irish church.
Having begun his ecclesiastical career at Derry, Gilla meic Liac maintained a keen interest in the affairs of that foundation. His circuit of Cenél nÉogain in 1150 coincides with the succession of Flaithbertach Ua Brolcháin (qv) as abbot of Derry, and it seems reasonable that the comarba Pátraic was involved in the appointment. From the perspective of Gilla meic Liac, it was important to bring the Columban church along in the process of reform. He was willing to facilitate Flaithbertach's advancement as comarba Coluim Cille (successor of St Colum Cille (qv)), or head of the Columban federation, a dignity held for the preceding 200 years by the abbots of Kells. Yet, in 1164, Gilla meic Liac blocked the ambitious abbot of Derry from succeeding to the vacant abbacy of Iona. At the same time the archbishop of Armagh obviously maintained a close working relationship with Derry, and there are indications that he facilitated the composition of an Irish life of Colum Cille: the text shows that the compiler had access to homiletic material at Armagh. He may also have been responsible for the updating of the Martyrology of Óengus (qv) (fl. c.830) and the composition of the Martyrology of Drummond, both of which have been assigned to Armagh in the early 1170s.
Gilla meic Liac's concerns for matters academic and spiritual were almost certainly matched by exigencies relating to the physical settlement at Armagh. While the only explicit reference to him in connection with building projects is the notice in 1145 of his construction of a lime kiln between the ecclesiastical site and Emain Macha, it was certainly the case that on three occasions during his episcopate Armagh was extensively damaged by fire and required reconstruction. This happened in 1137 (the year of his appointment), in 1164, and again two years later when, alone of the churches within the enclosure, the abbey of Saints Paul and Peter survived the conflagration. Armagh had long enjoyed the patronage of the Cenél nÉogain kings, and Gilla meic Liac could apparently count on the support of the high-king Muirchertach Mac Lochlainn. Muirchertach intervened in 1152, when Gilla meic Liac was physically attacked and wounded by supporters of the king of Airgialla, Donnchad Ua Cerbaill (qv), and had the latter deposed for a time. The archbishop, for his part, witnessed Mac Lochlainn's charter to the abbey of Newry. In 1165 Gilla meic Liac was guarantor for Mac Lochlainn's oath to spare the king of Ulaid, the breaking of which brought about the high-king's downfall. In spite of his having broken a solemn oath taken on the relics of St Patrick, the dead king's body was received by the archbishop into the church of Armagh. When the political order changed, and the English king Henry II (qv) asserted his overlordship of the Irish rulers, Gilla meic Liac made a pledge of fealty at Dublin in 1172. In common with many other Irish prelates at the time, he apparently believed that the king of England would ensure the further reform of the Irish church.
Gilla meic Liac was in his mid-eighties when he died 27 March 1174. His name was added to the list of those commemorated on 27 March in the Martyrology of Gorman (Máel-Muire Ua Gormáin (qv) (fl. c.1170)), where he is described as cenn Banba (Ireland's head). Gerald of Wales (qv), who reported the elderly archbishop's absence from the synod of Cashel in 1172, remarks that he was popularly believed to have been a saint, adding that he habitually travelled with a white cow in train, surviving solely on the milk of that animal. Gilla meic Liac's immediate successor in the see of Armagh was Conchobar son of Mac Con Caille, who died the following year; his sucessor in turn, Gilla in Choimded Ua Caráin, abbot of Saints Paul and Peter, held office until 1180.