He became co-King of Ailech with his brother Flaithbertach ua Néill on the death of his father in 943.
He became High King of Ireland on the death of his maternal cousin Congalach Cnogba of the southern Uí Néill sept of the Síl nÁedo Sláine.
Domnall is considered to have been an effective ruler who introduced military reforms later credited to Brian Boru.
[citation needed] Much of his reign was spent in war against his sister Dúnflaith's husband Amlaíb Cuarán, the King of Dublin.
Domnall was called "High King of Ireland" (ard-rí Érenn) in his obituary, and was the last of his family to hold the title.