Thomas Owen Clancy argues that Diarmait was one of the most important Céli Dé reformers, instrumental to the spread of the movement in Scotland, laying the ground for his successor Indrechtach.
Many anecdotes about the rule and philosophy of the abbot were recorded in the Monastery of Tallaght, for which Diarmait was probably one of the main sources and inspirations.
The same entry in the Annals of Ulster that reported Cellach's resignation, tells us that Diarmait was the alumnus, the pupil or fosterson of a man called Daigre.
[5] In 825, Iona was attacked by a group of Vikings,[6] but Diarmait lived on, as it is reported in 829 that he went again to Scotland with the relics of Colum Cille.
He is mentioned for the last time in 831, when the Annals of Ulster reported that "Diarmait came to Ireland with the halidoms (i.e. relics) of Colum Cille".