Diarmait (d. 825), grandson of Áed Rón, was founder and first abbot of Dísert Diarmata, a prominent member of the Céli Dé church reform movement, and a saint in the Irish tradition. His father, Fergal, was a relatively undistinguished member of the east Ulster Dál Fiatach dynasty; his mother, Clothbeo daughter of Conchobar, belonged to the minor lineage of Dál Conchobair. His grandfather Áed Rón (qv) (d. 735), however, was a powerful overking of Ulaid, while two of his uncles, Bressal and Fiachnae (qv), subsequently held the overkingship. In addition, his first cousin, Loingsech (d. 800) son of Fiachnae, was abbot of Downpatrick, a former royal site which Dál Fiatach established as an ecclesiastical centre in the mid eighth century.
A digression in the ‘Bóruma’ (cattle-tribute) tale purports to tell of Diarmait's young manhood and the foundation of Dísert Diarmata (Castledermot, Co. Kildare). It relates how the Laigin captured Diarmait and his companions while they were serving with the Uí Néill on a hosting into Leinster. Áed Rón, here represented as Diarmait's father, made an alliance with the Uí Néill whereby the young men were freed. Diarmait, abandoning dynastic ambitions, founded a monastery on land granted by Brandub (qv), overking of Leinster. The episode is completely anachronistic and accords with developments at the time the ‘Bóruma’ took shape in the eleventh century.
Historically, Diarmait was closely involved with the Céli Dé reform movement, which gathered pace in the second half of the eighth century. He is featured along with Óengus (qv) of Tallaght and others in the tract ‘The unity of Máel-ruain’. It seems that he spent a period as a monk at Bangor before establishing his dísert (hermitage) at Castledermot in 811. Castledermot was regarded as a daughter-house of Bangor; hagiographical sources associate Comgall (qv), the founder of Bangor, with Leinster and claim that he was granted lands in the Carlow area.
Diarmait died in 825, his distinction in learning acknowledged by his being styled religionis doctor in his obit. Castledermot, in spite of being raided at least twice by the vikings and having a Norse settlement nearby, survived as an ecclesiastical centre and was among the properties confirmed to the bishopric of Glendalough by Pope Alexander III in 1179.