O’Reily is born John O’Reilly on November 19, 1846, in Kilkenny, County Kilkenny, the son of Michael, a military officer, and Anne, née Gallagher. He completes his primary education at the parochial school of St. John’s Parish, and spends six and a half years at St. Kieran’s College. Due to poor health, he decides against pursuing a military career, and in 1864 he enters All Hallows College in Dublin to study for the priesthood. He learns the Irish language and studies mental philosophy, mathematics and ecclesiastical studies, achieving first prize in each of his classes.
After being ordained on June 21, 1869, O’Reily leaves Ireland for Western Australia in October, arriving in January 1870. Having served briefly in Newcastle (present day Toodyay) and Northam, he becomes a parish priest in Fremantle, establishing the West Australian Catholic Record in 1874 and serving as its publisher, editor and printer from 1883.
When the Diocese of Port Augusta is established in 1887, Pope Leo XIII names O’Reily as its first bishop. Concerned about the financial position of the diocese, which had inherited significant debt from the Diocese of Adelaide, he accepts the posting reluctantly. As bishop, he greatly improves the financial position of the new diocese, reducing its debt by half and earning a reputation as a competent administrator.
In 1894, O’Reily is appointed to replace the deceased Christopher Reynolds as Archbishop of Adelaide. The archdiocese he inherits is burdened with substantial debt, again left over from the old Diocese of Adelaide. Through the sale of church assets and a fundraising campaign, he is able to eliminate most of the Archdiocese’s liabilities while still investing in church infrastructure. He also actively participates in public discussions relating to education policy at a time when the role of the state in supporting religious education is topical. He publicly advocates government assistance for religious schools, stating that it is unfair Catholics pay taxes to support state schools, but receive no funding for their own.
In the later years of his life, poor health forces O’Reily to spend less time attending to his episcopal duties, and from 1905, he keeps to himself in his house in Glen Osmond, leading to the local press referring to him as the “Recluse of Glen Osmond.” Increasingly, his episcopal duties are fulfilled by Bishop of Port Augusta John Norton, who has to visit the more remote parts of O’Reily’s see on his behalf.
As he becomes more frail, O’Reily asks certain priests to accompany him when he travels, among whom is the Dominican prior Robert Spence. When O’Reily requests a coadjutor in 1913, he chooses Spence as his first preference for the role. Despite the reluctance of some clergy to the appointment of a religious as Archbishop, Spence is consecrated as coadjutor, with right of succession, in August 1914.
O’Reily dies at his house in Adelaide on July 6, 1915 and is buried under a large Celtic cross at the West Terrace Cemetery in Adelaide. He is highly regarded by many in South Australian society, with Adelaide’s daily newspapers praising his character, administrative ability and positive relations with non-Catholics.