After his primary education in Castletown-Geoghegan and Streamstown, he attended Saint Finian’s College, Mullingar. In September 1972 he began his studies for the priesthood at Saint Patrick’s College, Maynooth, obtaining a Bachelor of Science Degree in 1976, as part of his philosophical studies. During his theological studies he was awarded a Bachelor in Divinity Degree in 1979 and a licence is Theology in 1981, both by the Pontifical University, Maynooth. He was ordained to the priesthood in Saint Michael’s Church, Castletown-Geoghegan on 7 June 1980.
Upon the completion of his studies he was appointed as Curate in the Cathedral Parish of Christ the King, Mullingar. In 1985 he began doctoral studies in the Gregorian University and, in 1988, was awarded a Doctoral Degree in Theology. His final year in Rome also saw him serving as a Director of Formation in the Pontifical Irish College.
Following his return from Rome he was appointed Curate in Tullamore Parish and in 1989 lectured in Moral Theology at Saint Patrick’s College, Maynooth. In 1990 he was appointed Executive Assistant to the President of College; in 1993 he was appointed Vice-President of Saint Patrick’s College, Maynooth, and in 1996 was appointed President of the College, a position he held until his retirement in 2007.
From September 2007 until 2018 he served as Parish Priest of Dunboyne and Kilbride Parish, Co Meath, and was appointed Vicar General of the Diocese of Meath in 2009.
His appointment as Bishop of Ossory by Pope Francis was announced on 3 January 2018, and he was ordained bishop in Saint Mary’s Cathedral, Kilkenny, on 11 March 2018.
The Holy Father Pope Francis appointed Archbishop-elect Dermot Farrell as Archbishop of Dublin on 29 December 2020.
Bishop Farrell’s episcopal motto Adiutorium nostrum in nomine Domini (‘Our help is in the name of the Lord’) is taken from Psalm 124.