O'Reilly was born in Cork in the Diocese of Cork and Ross in 1952 and educated at both Scoil Chríost Rí and Coláiste Chríost Rí. In 1970, he entered the noviciate of the Society of African Missions in Wilton, Cork. He subsequently attended courses in philosophy and theology at St Patrick's College, Maynooth. In 1974 he was awarded a bachelor's degree in Literature, then in theology in 1977 and in 1978 the Diploma in missionary studies. On 10 April 1977, with the perpetual vow, he became a permanent member of the Society of African Missions. He was ordained a priest on 17 June 1978. After ordination he served for two years in pastoral ministry in the Archdiocese of Monrovia, Liberia. In 1980 he was sent to the Pontifical Biblical Institute, Rome, where he obtained in 1984, his Licentiate in Sacred Scripture. He was subsequently Professor of Biblical Studies at the seminary in Ibadan, Nigeria. From 1990 to 1995 he was a member of the Provincial Council of the Irish Province of his Institute, based in Cork and in May 1995 he was elected Vicar General. In 2001 he became Superior General of the Society of African Missions and was re-elected to a second term in 2007.
Episcopal Ministry
He was appointed Bishop of the Diocese of Killaloe by Pope Benedict XVI on 18 May 2010. The speed with which a successor to Bishop Walsh was named is notable as it normally takes a year to name a new diocesan ordinary. At the time of his consecration he was Ireland's youngest-serving Bishop in Ireland. He was consecrated on 29 August 2010. He was a popular and accessible bishop of Killaloe with one only public controversy when, in August 2014, he invited men to apply for the permanent diaconate in his diocese. Women, apparently outraged the introduction of this universal ministry of service, protested and O'Reilly backed down. On 22 November 2014, Pope Francis appointed O'Reilly to be the next MetropolitanArchbishop of the ecclesiastical province then denominated the province of Cashel, replacing retiring Archbishop Dermot Clifford. He took possession on 8 February 2015 of the metropolitan see of Cashel and Emly, established on 26 January 2015.
