It is vocative form of Eòghann. It’s not lenition, it’s slenderization (making the last consonant slender) or attenuation.
In general the vocative consists of a particle a and lenited vocative form of a noun (eg. a Mhàiri ‘oh, Mary’), but vowels do not undergo lenition, and the a particle disappears before vowels (so Iain or Anna are both nominative and vocative), and most masculine nouns slenderize in vocative:
- nom. Seumas, voc. a Sheumais (from this form comes English ‘Hamish’, btw.),
- nom. Eòghann, voc. Eòghainn.