Cellach was the son of Máel Coba mac Áedo (died 615) and belonged to the Cenél Conaill branch of the northern Uí Néill.
[1] According to the Irish annals derived from the Chronicle of Ireland, he may have been High King, jointly with his brother Conall Cóel, following the death of his uncle Domnall mac Áedo in 642.
Some historiographers say that four kings, namely Cellach and Conall Cóel and the two sons of Áed Sláine, namely Diarmait and Blathmac, ruled in shared reigns.All four putative successors to Domnall had been his allies at the great Battle of Mag Rath in 637, where Congal Cáech was defeated and the authority of the Uí Néill re-established, and it is not implausible that all four ruled together.
[5] In 650, they successfully ended a challenge to their authority in the north by defeating and killing their cousin Óengus mac Domnaill at the Battle of Dún Cremthainn.
[8] His daughter Cacht ingen Cellaig married Máel Dúin mac Máele Fithrich (died 681), King of Ailech.