[2] Son of Áed Findliath, Niall is first recorded succeeding his brother Domnall mac Áeda[2] as King of Ailech after the latter went into religious retirement in 911.
With a surge of Northmen forces the Uí Ímair launched an invasion in the south of Ireland in the years following 914 which greatly weakened the Eoghanacht of Munster and led to the establishment of a longfort at Waterford.
Intent on opposing this, and coming to the assistance of Munster, Niall's forces moved south and fought a large battle against a group of Northmen in the summer of 917,[2] ending inconclusively but with the death of many nobles and tributary kings in his following.
The army of the King of Leinster which had been called in to reinforce him however was utterly devastated near St Mullins, leaving the province in a tributary position to the Northmen.
[3] Despite his campaigns against the Northmen they continued to settle in large numbers, re-establishing their stronghold in Dublin (after a 15-year gap) and various ports on the eastern coast putting pressure on the Midland kingdom of Midhe.