Patrick O'Reilly was born at Rosscarberry, County Cork, Ireland, and baptised there on 24 February 1843, the son of Peter O'Reilly, a farmer, and his wife, Mary Keane. Peter and Mary O'Reilly brought their family to Auckland, New Zealand, in 1852. Patrick attended the boys' school attached to St Patrick's Cathedral and went to St Peter's select school, Auckland's first Catholic secondary school for boys. In 1858 he became a teacher at St Mary's College. The college, for which land had been granted by Bishop J. B. F. Pompallier, combined a seminary for the training of priests with a school for the education of Maori boys.
Deciding to become a priest, he completed his training at St Mary's, while continuing to work there as a teacher. By the time of his ordination on 24 February 1866, Patrick O'Reilly was a well-educated priest with a good knowledge of the French and Maori languages. He was one of the first Catholic priests to have received most of his early education and his training for the ministry in New Zealand. For the next five years he served at St Patrick's Cathedral as curate and continued to teach at St Mary's. The aftermath of the wars of the 1860s and the resignation of Bishop Pompallier brought about the closure of the college in 1869, and in 1871 Bishop Thomas Croke sent O'Reilly to the goldfield town of Coromandel. Here O'Reilly showed the organising ability that was one of his chief attributes. Within months he had built a church, St Coleman's, and about two years later he opened a school. In 1872 he baptised a new parishioner, Matthew Brodie, who was to become New Zealand's first native-born Catholic priest, and afterwards bishop of Christchurch.
In 1878 O'Reilly was moved to Thames parish where he was to spend most of the next 30 years. In addition to Thames, he had responsibility for Te Aroha and Ohinemuri parishes until 1882. O'Reilly's first priority was education. As Thames changed from a goldmining town to a farming centre, he guided his two parish schools – one for boys and one for girls – from small, financially precarious operations to stable and well-run institutions which were models of their kind. Between 1882 and 1896 this transition occurred under the direction of Bishop Edmund Luck. Luck was responsible for the reorganisation of Catholic education after the Education Act of 1877, which had excluded the possibility of financial assistance for privately run schools. O'Reilly was a close adviser of the bishop and had a hand in appointing the first diocesan inspector of Catholic schools. In 1900 he was honoured with the title of monsignor for this work.
On the departure of Bishop G. M. Lenihan for Europe in 1899, O'Reilly returned to Auckland to act as administrator in charge of St Patrick's Cathedral. The same year he was appointed dean of the Eastern District. In 1902 he was back in Thames. By this time O'Reilly was truly a patriarchal figure. His full black beard, stocky build and steady gaze reinforced the image.
Patrick O'Reilly remained parish priest of Thames until 1908, when he began to suffer from symptoms of mental deterioration. He spent most of the next six years in hospitals in Australia and Wellington and died at Ashburn Hall, Dunedin, on 25 August 1914. He was buried on 1 September at Otahuhu, Auckland.