The leader of the Catholic Church in Ireland, Cardinal Sean Brady, is to retire by 2015 at the earliest.
Monsignor Eamon Martin, administrator of the Diocese of Derry and a former teacher, has been named his successor and appointed assistant in the Archdiocese of Armagh during the transition.
One of his first roles will be to lobby for the church's anti-abortion stance as the Irish Government legislates to allow terminations on strict medical grounds.
After a wide-ranging address to parishioners in Armagh following the announcement, the 52-year-old said the church had to have the courage to speak out on issues.
"Sometimes it is a voice that people would prefer not to hear, particularly on the issue of abortion," Monsignor Martin said.
"But the protections for the equal right to life of the mother and the unborn child are already there and that could be clarified in the form of guidelines and made clearer so that none of our doctors and none of our nurses feel that they do not know or they would not be secure in doing what they believe is the best thing to protect life."
Cardinal Brady announced the appointment of a coadjutor archbishop outside St Patrick's Cathedral in Armagh after a scheduled morning Mass. It is a position he held in the 1990s when the Vatican moved to replace the late Cardinal Cahal Daly.
Monsignor Martin, former principal of St Columb's College in Derry, will celebrate a special Mass in Co Donegal on Saturday to show solidarity with those taking part in a pro-life vigil in Dublin.
He added: "We are at a crossroads in this country and we have to take a road forward which will protect and respect all human life from the first moment of conception until the moment of natural death."
Church sources revealed that the cardinal is expected to retire as archbishop and head of the church in Ireland in the next two to four years. Aged 73, he has led the church in Ireland for more than 16 years as a series of investigations exposed damning levels of clerical abuse.