He was born in Rosscarbery, County Cork, Ireland in 1843 and was brought by his parents to Auckland in 1852.

O'Reilly's major pastoral efforts were in Coromandel (where in 1864 he baptised Matthew Brodie, who was later the first New Zealand-born Catholic priest and who became the second Bishop of Christchurch) and Thames where he built churches and established schools which were "well-run institutions ... models of their kind".

O'Reilly assisted Bishop Edmund Luck in reorganising Catholic education in the Diocese after the passage of the Education Act 1977 which excluded the possibility of public financial assistance for privately run schools.

O'Reilly was appointed the first Diocesan Inspector of Catholic Schools and was raised to the rank of Monsignor for this work.

O'Reilly was administrator of St Patrick's Cathedral for a period from 1899 before returning as parish priest to Thames where he served until 1908.