In 1947 he went to National University of Ireland, Galway, on scholarship, took courses in Celtic Studies, archaeology, history and the classical languages, and graduated with an M.A.
[1] He was then admitted to the University of Bonn, Germany, for a PhD position and obtained his degree in 1955 for a dissertation on "Das Verbalsystem von Togail Troí (H. 2.
[1] In 1961, he co-founded Studia Hibernica, an academic journal for Irish studies published by the College, and for a time served at the editorial board (1961–1965).
[1] In the 60s, he also became President of Comhar na Múinteoirí Gaeilge,[1] which had just been founded to promote the use of the Irish language at schools, both as a teaching medium and a subject in itself.
Three years later, he became professor of Old and Middle Irish and Celtic philology at National University of Ireland, Galway, which would become the longest-held position of his career (1966–1994).
Mac Eoin has also been involved in editorial work for a number of journals and books, such as Studia Hibernica (volumes 1–5); Hereditas: Essays and Studies presented to Professor Séamus Ó Duilearga (with Bo Almqvist and Breandán Mac Aodha), Dublin, 1975; Proceedings of the Sixth International Congress of Celtic Studies, Galway, 1979 (with Anders Ahlqvist and Donncha Ó hAodha), Dublin, 1983; Proceedings of the Third International Conference on Minority Languages, (with Anders Ahlqvist and Donncha Ó hAodha), Clevedon, 1986; Celtic Cultures Newsletter (UNESCO), Nos.