He was the son of Fáelchar Ua Máele Ódrain (died 693), a previous king.
[1] The dynasty that ruled over Osraige in the early Christian period was known as the Dál Birn.
At this time Osraige was part of the Kingdom of Munster and in the 8th century was often involved in conflicts with the men of Leinster, the Laigin.
The location of the battle makes it probable that Cellach of the Osraige was the aggressor and was taking advantage of the rivalry of the Uí Dúnlainge and the Uí Cheinnselaig branches of the Laigin at this time.
[3] His son Dúngal mac Cellaig (died 772) was also King of Osraige but the kingship became dominated by a different line.