Cormac's first voyage was from Irrus Domnann on the coast of north-west Mayo. The second involved a journey to the Orkneys, then part of the kingdom of the Picts, for which a safe passage was negotiated by Colum Cille (qv) (d. 597) with Bruide, king of the Picts. He later returned safely to Iona. Adomnán's account (ii, 42) of Cormac's third voyage of fourteen days ‘to an area under the most northerly skies’ gives an unusual description of the sailors being assailed by a swarm of spiny creatures the size of frogs swimming in the sea. If, as seems likely, it is based upon a written source, it preserves one of the earliest examples of immrama or voyage-tale literature. Colum Cille had a vision of the predicament of Cormac and his companions and prayed with the brethren on Iona for their safe deliverance. Adomnán states (iii, 17) that Cormac was present on the island of Hinba, in Argyll, at a mass celebrated by Colum Cille in the presence of Brendan (qv) (d. 577/83) of Clonfert, Comgall (qv) (d. 602) of Bangor, and Cainnech (qv) (d. 600/603) of Aghaboe, which places Cormac around the third quarter of the sixth century.
Cormac was a monk of the Columban familia and is associated in Irish sources with Colum Cille's earlier foundation at Durrow. The notice in the Martyrology of Tallaght (21 June) gives him as ‘Cormac Ua Liatháin in Durrow’. Adomnán states (iii, 17) that he also founded monasteries of his own. Cormac is the subject of a number of Irish poems, most of which concern his special relationship with Colum Cille.