Tradition records that Cináed was born half-blind to his mother Muirenn (died 748), after Saint Adomnán cursed his father Írgalach's descendants for the killing of Niall mac Cernaig Sotal (died 701) of the rival Uí Chernaig sept of South Brega.
The Fragmentary Annals of Ireland render the words of the saint as follows:[2]"The infant in your womb will be king indeed, but one of his eyes is now broken as a result of the cursing of his father.
His accession to the rule of North Brega could date from 718 when he succeeded Amalgaid mac Congalaig (died 718).
In 725 is recorded his taking of Ulaid, and in 726, he defeated the Leinstermen at the Battle of Maíne and he received what he wanted from the Laigin.
[8] With the exception of Congalach Cnogba in the 10th century, he was the last Síl nÁedo Sláine High King of Ireland.