Máel Sechnaill
Cormac's family had been rulers of the Kingdom of Mide since at least the early 6th century. Known as the Clann Cholmáin, they were a branch of the southern Uí Néill dynasty. They had taken the surname Ua Mael Sechlainn after their prestigious ancestor, Máel Sechnaill mac Maíl Ruanaid, who reigned as High King of Ireland from 845 to 860. One of his descendants, and another of Cormac's ancestors, was Máel Sechnaill mac Domnaill (known as Máel Sechnaill II) who was High King before and after the reign of Brian Boru. Following the reign of Máel Sechnaill mac Domnaill from 976 to 1022, the Kingdom of Mide was quickly reduced to a fraction of its former power.