It is not certain if he ever existed, as the only source, Betha Chellaig, is a much later pseudo-historical account found in Leabhar Breac.
The account states he was the eldest son of Eogan Bel, King of Connacht, was taught by Ciarán of Clonmacnoise.
Ciarán made him a monk, and Cellach stayed with him until Eogan Bel was killed in the battle of Slicech (Sligo) against the northern Uí Néill, dated at 543, 546 or 547.
Cellach was eventually ousted and returned to Ciarán, with whom he remained until elevated to bishop of Killala in the reign of Tuathal Maelgarb.
Cellach's brother, Muiredach, is said to have avenged his death, and the four murderers buried under the Dolmen of the Four Maols, outside Ballina.