Congal is presumed to have become king of the Dál nAraidi in 626 following the death of Fiachnae, but he is unlikely to have ruled as king of the Ulaid until some time after the death of Fiachnae mac Demmáin in 627. He first appears in the record in 628, when he killed Suibne Menn of the Cenél nEógain, supposedly High King of Ireland, at Traig Bréni on the shore of Lough Swilly. This killing may have opened Congal's way to becoming king of the Ulaid, but it also brought Domnall mac Áedo of the Cenél Conaill, Congal's nemesis, to the headship of the Northern Uí Néill. According to the Fled Dúin na nGéd, Domnall was the foster-father of Congal. Domnall had clashed with Suibne earlier that year and it is possible that Domnall and Congal were acting in concert.