Diarmait (d. 852), abbot of Armagh, was grandson of one Tigernán. He contested the abbacy with Forannán (qv) (d. 852) son of Murgal, after the death in 834 of Abbot Éogan Mainistrech (qv). In common with his opponent, Diarmait was acknowledged as a man of learning and he was acclaimed as ‘the wisest of all the doctors [of theology] of Europe’. Yet for the next seventeen years the control of Armagh remained in dispute between them.
Initially forced to give way to his rival, Diarmait gained the support of the powerful Munster king Fedelmid (qv) son of Crimthann, who was already making his mark on Armagh politics. While the latter impeded the efforts of Forannán to establish his claims, Diarmait made a successful circuit of Connacht, bringing with him three important Patrician relics: the Bachall Ísu (‘staff of Jesus’, believed to have been a crosier), a bell, and a gospel-book associated with St Patrick (qv). In 839 Diarmait was installed at Armagh, and it seems that some power-sharing arrangement was worked out between the rivals. Both Diarmait and Forannán made a visitation of Munster in 842 and proclaimed the Law of Patrick. Perhaps Diarmait took advantage of the situation when, three years later, Forannán was captured in Munster.
In any event, by 848 Diarmait had finally managed to displace his opponent; the succession list of coarbs (successors) of Patrick accords Diarmait a four-year term of office. It may have been during this period that he augmented the termonlands of Armagh, making a grant of lands in the vicinity of Cross Ardachaid (LL, 42d). In 851 Diarmait, as coarb of Patrick, played a prominent role in the rígdál (royal meeting) at Armagh between the king of Tara – Máel-Sechnaill (qv) son of Máel-ruanaid – and Matudán, overking of Ulaid. Accompanying Diarmait on this occasion was a leading member of his community, Fethgna (qv) son of Nechtan. Diarmait died in 852, the same year as his rival Forannán. Both men are styled heredes Patricii (heirs of Patrick) in their obits (AU). It appears that the abbacy of Armagh passed to Fethgna at this point.