Little is known of his early history; it is generally supposed that he was educated at Iona, and that, having embraced the monastic rule, he returned to his own country to extend its observance there.

He presided over the great Abbey of Raphoe, of which he was the founder, until the year 679, when he was raised to the government of his order, and from that period he usually resided at Iona.

The fact of his having been chosen to such an important office, is a sufficient testimony to his virtues, and of the veneration and respect in which he was held by his contemporaries.
St.Adamnan paid more than one visit to his friend the Northumbrian monarch (_regem Alfridem amicum_).

On the second occasion he went with the Abbot Ceolfrid, and after some conversation with him and other learned ecclesiastics, he adopted the Roman paschal computation.

Yet, with all his influence and eloquence, he was unable to induce his monks to accept it; and it was not until the year 716 that they yielded to the persuasions of Egbert, a Northumbrian monk.