According to the Lebor Gabála Érenn, he took the throne after his father was overthrown by Caicher son of Nama, brother of Nechtan.
It is suggested that he is connected to the presumed deities (or possibly two names for one deity) Nuada Airgetlám and Nechtan of the Tuatha Dé Danann.
Cormac's successor Nechtan was bishop by at least 1131, when he appears in a charter recorded in the Gaelic notitiae on the margins of the Book of Deer.
John of Fordun claims that Drest reigned for 45 years in the time of Palladius rather than Patrick, and conflates him with his brother Nechtan.
Moreover, Gavin Dunbar, a 16th-century Archbishop of Glasgow, wrote in his Epistolare that Nechtan's see was moved from Mortlach to Aberdeen in the year 1125, partially contradicting the account of Boece.