Éogan Mainistrech (‘of the monastery') (d. 834), abbot of Armagh, Clonard, and Monasterboice, was probably of midlands origin. His father had the rare personal name of Ainbthech, which is attested among the Delbna. Éogan himself may well have commenced his career as fer léigind (lector or chief scholar) of Monasterboice, a role he exercised according to the later annals. His advancement was no doubt assisted by the fact that he became the anamchara (spiritual adviser) of Niall Caille (qv), king of the Cenél nÉogain and later king of Tara. The description of Éogan as comarbae Buite (coarb or successor of St Buite (qv)) suggests that he was head of the community at Monasterboice, even though he is not described as such in his obit.
With the support of Niall Caille, Éogan succeeded Abbot Flandgus son of Loingsech at Armagh when the latter died in 826. According to the abbatial list, he held office for eight years, although his claim to the position did not go unchallenged. He was opposed by Bishop Artrí (qv), possibly a son of the then Clann Cholmáin king of Tara, Conchobar (qv) son of Donnchad Midi (qv), who had the support of the powerful Munster overking, Fedelmid (qv) son of Crimthann. It may be significant that Fedelmid plundered the small midland kingdom of Delbna Bethra that same year. It seems that Éogan was temporarily displaced; the lists credit Artrí with having held abbatial office for two years.
The battle of Leth Cam near Armagh in 827, however, confirmed Cenél nÉogain dominance of the foundation and secured Éogan's tenure of the abbacy. His lay patron, Niall Caille, soundly defeated Cummascach son of Cathal, mesne-king of Airgialla, and Muiredach son of Eochaid, overking of Ulaid, both of whom were clearly concerned at what they saw as the extension of intrusive interests at Armagh. Éogan's victory at the expense of Artrí may have been one reason (although clearly not the only one) for the rígdál (royal meeting) between Fedelmid and Conchobar son of Donnchad that same year. In any event, the latter attacked Éogan in 830, capturing his retinue and stealing his cattle.
Shortly afterwards, on the death of Cormac son of Suibne, Éogan succeeded to the abbacy of Clonard. He died in 834, and the succession at Armagh was then disputed for seventeen years by Diarmait (qv) grandson of Tigernán and Forannán (qv) son of Murgal, a bishop; these two rivals died in 852.