Training as a Marist priest, he was educated at the
University of Passau, Germany, and the
Milltown Institute, Dublin. Marmion also studied at
Heythrop College, University of London,
All Hallows College, Dublin,
Trinity College, Dublin and the
Catholic University of Leuven, Belgium. He lectured in theology at the
Milltown Institute of Theology and Philosophy, before joining the staff of Maynooth College in 2013. He served as Dean of theology from 2015 to 2021. Marmion has written on theology and the church, and contributed to many publications, and academic journals including
Louvain Studies and
Milltown Studies, and also written articles for the Irish Catholic and the Irish Times. Marmion has been the chief editor of the
Irish Theological Quarterly since 2013. He also serves as one of the series editor of
Studies in Theology, Society and Culture, published by
Peter Lang. ==Comments at funeral of Sean Fagan==