John D'Alton was born in Claremorris to Joseph D'Alton (d. April 1, 1883) and his wife Mary Brennan, at the height of the Land Wars in Ireland. He was baptised four days later, on October 15, 1882, with Michael and Mary Brennan acting as his godparents. D'Alton's mother had a daughter, Mollie Brennan, from a previous marriage; she remarried again after the Cardinal's father died in 1883. He died from a heart attack in Dublin at age 80, and was buried on the grounds of St Patrick's Cathedral. He was succeeded by his auxiliary bishop, William Conway.