Monsignor Eamon Martin has said he was "shocked" and "humbled" when he found out the Pope had nominated him as Coadjutor (assistant) Archbishop to the Archdiocese of Armagh.
Monsignor Martin, 52, currently leads the Diocese of Derry, in his caretaker role as diocesan administrator.
He was elected as administrator in November 2011, following the retirement of the Bishop of Derry, Seamus Hegarty, who stepped down on health grounds.
In a statement on Friday, the new coadjutor archbishop said he was "humbled and honoured" by the appointment, but also admitted that he had accepted the role with "considerable nervousness and trepidation".
If, as expected he becomes Dr Brady's successor, he will face formidable challenges; the legacy of clerical child abuse in the Irish Catholic Church, the reform of abortion legislation in the Republic, falling church attendance figures and a sharp drop in ordinations to the priesthood.
Eamon Martin was born in Derry in 1961 to parents who were both originally from County Donegal in the Irish Republic.
He was one of 12 children - six boys and six girls - and told reporters on Friday: "There was complete equality in our house."
He grew up in Pennyburn in the city where he attended the nearby St Columb's College.
He later studied for the priesthood in Maynooth, County Kildare, and was ordained in 1987 at the age of 26.
His first ministry was in the parish of Templemore, Derry.
In 1990, he joined the teaching staff of his former school, St Columb's College where he taught mathematics and religion.
He became president of the college in May 2000 and later became secretary of the Irish Episcopal Conference.
The new coadjutor archbishop is also a director of the National Board for Safeguarding Children - the body set up in the wake of widespread clerical abuse scandals in the Irish church.
On his appointment, he said one of the greatest challenges facing the church was to live with and learn from the past.
He told a press conference in Armagh that the church "can never take it for granted that the safeguarding systems we have in place are robust and failsafe, so we have to keep working on that".
He described himself as both "fearful" and "enthusiastic" about the role he had accepted.
Monsignor Martin is also member of both the executive committee of the Catholic Heads' Association and the Northern Ireland Commission for Catholic Education (NICCE).
Early last year, he published a set of proposals outlining a vision for the reform post-primary education in Derry.
The paper, Together Towards Tomorrow, put forward a number of proposals including the abolition of academic selection and co-education instead of single sex schools.
He has degrees in mathematical science from National University of Ireland (NUI) Maynooth, theology from St Patrick's College, Maynooth and an MPhil in school development from the University of Cambridge.
Monsignor Martin has a keen interest in music and has served as director of music at St Eugene's Cathedral, where he was ordained and where he also ministered.
He is also a regular contributor to the Thought for the Day slot on BBC Radio Ulster.