Cormac belonged to a minor branch of the Eóganachta clan which dominated Munster in the 8th and 9th centuries. According to genealogies, he was a member of the "Eóganacht Chaisil", the Cashel branch of the clan. This kin group was important, but Cormac came from a very minor branch. He was considered to be an eleventh generation descendant of Óengus mac Nad Froích and none of his ancestors since Óengus were counted as kings of Cashel. For this reason, it is most likely thought that Cormac, as well as other 9th century kings of Munster who were bishops and abbots, was a compromise candidate.