 He ruled from 770 to 788.His uncle, the high king Niall Frossach (died 778), abdicated in 770 or 772 at which time Máel Dúin became King of Ailech.Expeditions by the new high king, Donnchad Midi (died 797) of Clann Cholmáin of the southern Uí Néill, to assert his authority in the north are recorded in 771 and 772.