Cormac (846–908), son of Cuilennán and king of Cashel (seat of the Éoganacht kings of Munser), was according to the genealogies son of Sealbach, grandson of Dub-dá-chrích, and descendant of Óengus (qv) son of Nad-fraích. He was the chief contender for Éoganacht claims to the high-kingship of Ireland against the Uí Néill of Tara. When he became king of Cashel in 902 (Ann. Inisf. 901), he is described as ‘noble bishop and celibate’, though not as bishop of Cashel. As a member of the Clann Fhaílbe branch of Éoganacht Chaisil, his dynastic pedigree is a long way distant from previous kings of Cashel, so that his succession has been conjectured to be a compromise choice against rival claimants, because of his piety and religious vocation. His aspirations to the high-kingship and the hopes of Éoganacht Chaisil were ended by his defeat at the hands of Flann Sinna (qv), king of Tara, at the battle of Belach Mugna (near Leighlinbridge, Co. Carlow) in September 908. A graphic account is given of his death in the Fragmentary Annals.
Cormac is credited with the authorship of ‘Sanas Cormaic’ (Cormac's glossary), a compendium of explanations and etymologies of obscure or archaic words, which survives in several manuscripts, including the Book of Leinster. He also possibly compiled the lost Psalter of Cashel, a collection of genealogical material, tribal origin-tales, and other historical matter (including a poem on the Leinster kings up to the beginning of the ninth century), now surviving only in fragmentary late copies.