Cellach (d. 658), son of Máel-Cobo and high-king of Ireland, belonged to the dynasty of Cenél Conaill. His father Máel-Cobo (d. 615), one of the five sons of Áed (qv) son of Ainmere (qv), was high-king according to some sources; his father's only recorded wife was Cróinseach, daughter of Áed Finn king of Osraige (Lec. 390b). Cellach's immediate predecessor in the kingship of Cenél Conaill and in the high-kingship was his uncle, Domnall (qv) (d. 642) son of Áed. Although Cellach appears in Middle Irish regnal lists (e.g. Rawl. B. 502, 137a), his absence from the earlier ‘Baile Chuinn’ raises questions about the reality of his achieving the high-kingship. In any case, it would appear that for most of his reign Cellach shared power with his brother Conall Cóel (qv) (d. 654). In 650 (AU) the two brothers faced a challenge from their first cousin, Óengus son of the above-mentioned Domnall. They defeated and slew Óengus at the battle of Dún Cremthainn.
Cellach was married to Dathnat or Damnat, whose dynastic origin is unknown (Lec. 390b; RIA, D.2.1, 96rb). They had a daughter Cacht, who married Máel-dúin, king of Cenél nÉogain, and became the mother of a future high-king, Fergal (qv) son of Máel-dúin. It seems likely that Cellach was also the father of Findelb, wife of the later high-king Sechnassach son of Blathmac (qv) and grandson of Áed Sláine (qv) (Lec. 391; RIA, D.2.1, 97). There is no record, however, of Cellach having left male issue; on his death, the high-kingship passed to Blathmac son of Áed Sláine, while the kingship of Cenél Conaill continued in the line of his uncle Domnall.