Archbishop Kieran was born in Cork City on 8th August 1952, the first of five children born to Sean and Theresa O’Reilly. He attended Scoil Chríost Rí and Colaiste Chríost Rí for his primary and secondary education.
In 1970 Archbishop Kieran began his studies for the priesthood with the Cork-based Society of African Missions. Following university studies in St. Patrick’s College , Maynooth, he was ordained to the priesthood on 17 June 1978 in the African Missions Church at Wilton, Cork. He was then appointed to pastoral work in the Archdiocese of Monrovia, Liberia.
Two years later, the future archbishop was recalled to undertake studies in Sacred Scripture at the Pontifical Biblical Institute, Rome, from where he qualified with a licentiate in Sacred Scripture. His next appointment was to the Major Seminary of SS. Peter and Paul, Ibadan, Nigeria, where he lectured in Sacred Scripture for five years. Then, in 1989, the members of the Irish Province of the SMA elected him a member of the Provincial Council, the body which oversees the administration of the Society in Ireland.
Six years later, at the 1995 SMA General Assembly in Rome, Archbishop Kieran was elected the SMA Vicar General. As a member of the General Council, he was responsible for several matters – formation of seminarians and priests; justice and peace issues and the development of the SMA in Africa. In 2001 he was elected the eleventh Superior General of the Society of African Missions a position he held until 2010 when he was appointed Bishop of Killaloe.
Archbishop Kieran attended three Synods of Bishops – on the Word of God (2008), the Second Special Synod for Africa (2009) and the Synod on New Evangelisation (2012).