Finnian studied in Wales under Sts. David, Cadoc, and Gildas before returning to Ireland and founding around 520 the great monastic school of Clonard on the Boyne in modern County Meath. Clonard under his rule became the principal Irish monastic school of the 6th century, with a community of three thousand at its height.
Among his many pupils were the famous 'Twelve Apostles of Ireland' — St. Columba of Iona, St. Brendan of Clonfert, St. Ciarán of Clonmacnoise, St. Brendan of Birr, St. Senan of Iniscathy, St. Mobhi of Glasnevin, St. Ruadhán of Lothra, St. Ninnid of Loch Erne, St. Lasrén of Devenish, St. Cainnech of Aghaboe, St. Laisrén MacNadfraich, and St. Colmán of Dromore — every one of whom founded a major monastery of his own. Through Finnian the Irish monastic golden age of the 6th and 7th centuries took its character.
His penitential — a list of penances assigned for various sins — circulated widely and influenced the early Irish Penitentials that, through Cassian and Columbanus, gave the Western Church the practice of private auricular confession that displaced the older public penitential discipline.
He died at Clonard in the plague of 549 along with many of his community. The abbey was destroyed by Vikings in the 9th century and again at the Reformation.