Airechtach (d. 794), abbot of Armagh, belonged to the line of Fáelán within the ecclesiastical lineage of Uí Bressail, which in turn represented a discard segment of the ruling dynasty of Airthir (baronies of Upper and Lower Orior, Co. Armagh). He was a priest at Armagh when Affiath was bishop.
According to the Annals of Tigernach (Tigemach Ua Bráein (qv)), Airechtach was responsible for the battle of Emain Macha (759). The previous year the incumbent abbot Céle Petair, who also belonged to Uí Bressail, had died. It seems that Airechtach, who was probably then in his thirties, considered that he was entitled to succeed his kinsman in the abbacy. Instead the office was accorded to Fer-dá-Chrích of the rival lineage of Uí Nialláin. Bitter rivalry between the two led to violence as their dynastic connections were drawn into conflict. Battle was joined at Emain Macha (the ancient royal site of the Ulaid, near Armagh) between the Ulaid and the Uí Néill, in which the Ulster king Fiachnae (qv) son of Áed Rón (qv) was victorious; two dynasts of the opposing side, Dúngal grandson of Conaing, and Donn Bó son of Cú Brettan, were slain. Fer-dá-Chrích remained abbot.
Airechtach was again passed over for the abbacy when Fer-dá-Chrích died (768). He finally secured the office in the last year of his life, after the death (793) of Dub-dá-Leithe (qv), son of Sínach. Apparently by coincidence, Airechtach died on the same night as Bishop Affiath.