 John O'Brien attended the Christian Brothers' school in Kilrush until entering the junior seminary for the Diocese of Killaloe in Ennis when he was sixteen.He successfully completed his Leaving Certificate and, not being the chosen for the seminary of Killaloe, O'Brien was directed instead to the Archdiocese of Dublin, where he began his studies in Clonliffe College in September 1949.He was ordained a priest by Archbishop John Charles McQuaid on 25 May 1956.