Mac Carthaig, Cormac (d. 1138), son of Muiredach, king of Desmumu (Desmond) and overking of Munster, belonged to the Clann Donngaile lineage of Éoganacht Chaisil. His father, Muiredach (d. 1092), was a son of the eponymous Carthach, who in turn was a great-grandson of Cellachán Caisil (qv), overking of Munster. His mother was Findatail, daughter of Conchobar Ua Cathaláin, a minor dynast of Uaithne Cliach (barony of Owney, Co. Tipperary). Cormac had at least two brothers, Tadc (d. 1124), who preceded him as king of Desmumu, and Donnchad (d. 1144), and also a sister, Ben-Laigen. In turn, he married Derbail, daughter of Ua Lorcáin, an Uí Muiredaig dynast of northern Leinster, who was the mother of his son Diarmait (qv) – dubbed Diarmait Cille Badhúna (probably relating to Kilbane, parish of Desertmore, Co. Cork) by later genealogists.
As a young man, Cormac was present in 1118 at an assembly near Glenn Maidir (Glanmire, Co. Cork) at which his brother Tadc with the support of the overking of Connacht, Tairdelbach Ua Conchobair (qv), and the king of Mide, Murchad Ua Máelshechlainn (qv), took a stance against the supremacy of Muirchertach Ua Briain (qv), which finally ended the latter's claim to the high-kingship of Ireland. In 1123, his brother Tadc having fallen seriously ill, Cormac took the kingship of Desmumu in his presence, backed by several of his most powerful vassals, including Ua Mathgamna of Uí Echach Muman. In the first five years of his reign Cormac engaged in a contest with Ua Conchobair of Connacht, now the leading claimant to the high-kingship, which almost resulted in his political demise. Following his brother's death in 1124, Cormac banished Ua Muirchertaig of Lough Leane and two other Munster sub-kings. He then chose to support a revolt by the overkings of Leinster and of Mide, Énna Mac Murchada and Murchad Ua Máelshechlainn, against the supremacy of Ua Conchobair, taking part in a hosting as far as Innéoin (Dungolman, Co. Westmeath).
Cormac found his authority in Munster undermined when the overking of Connacht took the initiative against him from 1125 onwards, employing the exiled Ua Muirchertaig of Lough Leane to harass the west coast of Munster with a naval fleet. Although he asserted his authority over Limerick, Cormac's principal allies were defeated by Ua Conchobair; Ua Máelshechlainn was banished from his kingship and Mac Murchada was forced to yield hostages. Despite Cormac's efforts to block Ua Conchobair's forces in Osraige, the latter pushed into the heartland of Munster, reaching Móin Mór in the latter part of 1126, and taking great quantities of cattle and of booty. In parallel with this, Ua Muirchertaig returned to pillage coastal Munster, sailing via Castlemaine Harbour up to Lough Leane itself.
As Munster struggled against adversity, early in 1127 Cormac was deposed by a party of his own nobles. He withdrew to the religious life at Lismore, where, it is claimed, he was influenced by St Malachy (qv), while his brother Donnchad took the kingship in his place. Within months, the political situation had deteriorated further; a hosting by Ua Conchobair reached Cork, and Donnchad was forced to submit and yield hostages. At Lismore, Cormac was approached by the king of Tuadmumu, Tairdelbach Ua Briain (qv) (d. 1167) and his brother Conchobar (qv), and invited to retake the kingship of Desmumu. On his restoration, one of his first actions was to banish Ua Muirchertaig and two of his own nephews, sons of Tadc Mac Carthaig. Following two attempted naval incursions against west Munster by the son of Ua Muirchertaig on behalf of Ua Conchobair, Cormac and his ally Ua Briain placed a fleet on Lough Derg. It seems that a peace with the men of Connacht was reached around 1128.
The agreement achieved with Ua Conchobair was abruptly terminated in 1131, when Cormac's brother Donnchad brought a fleet to harry the coast of Connacht. Cormac was now militarily well prepared, and took the offensive by land and sea. Early in 1132 he destroyed the fortress of Bun Gaillme (at the mouth of the Galway river), and defeated and killed one of Ua Conchobair's vassals, Conchobar Ua Flaithbertaig, king of Iarthar Connacht, at An Cloide (in Co. Galway). This time, the war went in Cormac's favour; in 1134 he went on campaign with his Ua Briain allies in central Connacht. At Killeely, Co. Galway, they cut down the Ruadbethech, a great tree once held sacred and which retained importance in the traditions of Ua Conchobair's dynasty. They sacked several forts, including the stronghold at Athlone, and killed a kinsman of Tairdelbach, one Cathal Ua Conchobair, in battle. Later the same year, having assembled an even larger land and naval force, which included Munster, Leinster, Hiberno-Scandinavian and Mide contingents, Cormac was dissuaded from invading Connacht only by the intervention of the archbishop, Muiredach Ua Dubthaig.
No sooner had Cormac achieved a favourable resolution to the conflict with Ua Conchobair, than disagreement broke out between Éoganacht Chaisil and their erstwhile Ua Briain allies. In an ensuing skirmish two Ua Briain dynasts were slain. In 1135 Cormac campaigned in Tuadmumu with mixed results. The following year Ua Briain plundered Cell Íte (Kileedy, Co. Limerick) and Ráith Maige Desceirt (Ratass, Co. Kerry), but Cormac retaliated, killing a nephew of Ua Briain in battle, along with a dynast of the Déisi of Munster who supported him. He then laid claim to the overkingship of Munster and of Osraige.
In parallel with his military ventures, Cormac was a major patron of the church, spending lavishly on ecclesiastical building programmes and helping to promote reform. In the early 1120s, before he had yet succeeded to the kingship, he and his brother Tadc had jointly sponsored the enshrinement of the arm of St Laichtín (qv). As king, he helped to foster connections with the Irish Benedictines at Regensburg in Germany, where his kinsman Christianus Mac Carthaig (qv) was a member of the community. He may also have encouraged links with the reforming party at Canterbury. Following his restoration in 1127, he ordered the construction of the elaborate church known as ‘Cormac's chapel’ on the Rock of Cashel; it is in a Romanesque style which, it is claimed, displays both German and English influences. Cormac is credited with having built twelve other churches, at Lismore and other locations. One of these was the monasterium Ibracense (apparently Ballinskelligs, Co. Kerry), which was occupied by Canons Regular of St Augustine.
Cormac achieved much in both political and ecclesiastical spheres, but in 1138 his career came to an untimely end. Having blocked a hosting to Waterford led by Ua Briain the previous year, Cormac was assassinated in his house at Mag Tamnach (Mahoonagh, Co. Limerick) by two of his own retainers, at the instigation of Ua Briain. His obit acclaimed him as ‘the most pious and valorous of men’. His immediate successor was his brother Donnchad, who continued the struggle against Ua Briain; having been betrayed to his great rival, however, Donnchad died in captivity in 1144. He was succeeded by Diarmait son of Cormac, who reigned till 1185; most of the later Mac Carthaig kings of Desmumu descended from Diarmait.