Cormac mac Cuilennain was a member of the Eoga-nachta Chaisil branch of the Eoganachta, though like Fedelmid mac Crimthainn (d. 847), no ancestor of his had been king of Munster since Oengus mac Nad Fraofch, grandson of the legendary founder of the Eoganachta dynasties, Corc of Cashel, direct ancestor of the most successful eastern Eoganachta branches, whose death is mentioned in the annals around 489. Cormac mac Cuilennain became king of Munster in 902 and may have been a compromise candidate in the absence of strong opposition from the main branches of the dynasty. From his early years he is reputed to have been of a scholarly and pious nature, and may have been ordained priest and bishop, though this cannot be verified. Although of an ascetic nature, he is said to have been betrothed or even married to Gorm-laith, daughter of Flann Sinna mac Maele-Sechnaill (southern Uf Neill king of Tara from 879 to 916), but rejected her because of his wish to remain celibate. It may be noted that celibacy was not a requirement for high office in the Church at that time. Gormlaith was then married to Cerball mac Muireccain, king of Lein-ster, who is said in a bardic poem to have treated her badly, and later very happily married to Niall Glundub, northern Uf Neill king of Tara who was killed by the Vikings at Islandbridge, Dublin, in 919. Serial marriage was not unusual among women of noble birth in medieval Ireland, in a society that sanctioned divorce and used marriage as a means of cementing alliances.
Cormac mac Cuilennain is credited with several scholarly works, among them genealogical tracts for the whole of Ireland in which Eber son of Mil, ancestor of the Eoganachta, comes first instead of Eremon, ancestor of the Uf Neill. The most famous extant work assigned to him is the Sanas Chormaic, a glossary containing etymologies and explanations of over 1,400 obsolete and difficult Irish words that may have been part of the lost Psalter of Cashel, a compilation of origin tales, genealogies, and tribal histories, part of which may be found in the MS Laud Misc 610 (now in the Bodleian Library). The fragment that has survived contains Munster origin tales and tribal histories, as well as the aforementioned genealogical tracts.
As the Uf Neill continued to threaten the sovereignty of Munster, Cormac was the last Eoganachta king of Munster to challenge northern hegemony. In 906 the southern Uf Neill king of Tara, Flann Sinna, assisted by the king of Leinster, Cerball mac Muire-chain, led his forces into Munster and was met and defeated by the Munstermen under Cormac, at Mag Lena (modern Tullamore, Co. Offaly). In 908, Flann Sinna, once again with the crucial assistance of Cerball, king of Leinster, and Cathal, king of Connacht, returned to the attack, as Cormac—instigated, according to an eleventh century text, by Flaithbertach mac Inmainen of the Muscraige, Abbot of Inis Cathaig— claimed tribute from Leinster and is said to have signified his intention of assuming the position of high king. The text was written in the interests of the Osraige, neighbours and former vassals of the Eoga-nachta, and there is unlikely to be much truth in it. It is more likely that Cormac’s intentions were to discourage the Uf Neill from further attacks on Munster. In a battle fought at Belach Mugna near Leighlinbridge in County Carlow, the Munstermen suffered a complete defeat and Cormac was killed in the battle. He was succeeded by Flaithbertach mac Inmainen, the last king of Munster to be a cleric. The practice of elevating clerics to the kingship is unique to the south of Ireland, although not to Munster, as it occurred in south Lein-ster also. With the death of Cormac mac Cuilennain, the decline of the Eoganachta overkingship, which had begun in the previous century, became more pronounced, and it was replaced by the Dal Cais, in the person of Brian Boru, in 978.
