Mannix was the son of a tenant farmer near Charleville, in County Cork, and was educated at Irish Christian Brothers schools and at St Patrick's College, Maynooth seminary, where he was ordained as a priest in 1890.
Mannix was the son of a tenant farmer near Charleville. This was in County Cork. Mannix was educated at Irish Christian Brothers schools and at St Patrick's College seminary. St Patrick's College was Maynooth. He was ordained as a priest in 1890.