Irish Catholic Cardinal Cahal Daly is in a serious condition in Belfast's City Hospital.
Cardinal Daly is in the hospital's coronary care unit suffering from a a serious heart condition.
The 92-year-old cardinal was ordained as a priest in 1941 and retired in 1996 when he was the Primate of All Ireland.
Since his retirement he has published many books and papers on the subject of philosophy.
Cardinal Daly is best known for leading a delegation on the Forum for Peace and Reconciliation in the 1990's and the New Ireland Forum in the 1980's.
An advocate of peace, he spent much of his time seeking an end to violence in the North of Ireland by peaceful means.
The Cardinal strongly condemned the IRA's campaign during the Troubles and strongly voiced his support for the Downing Street declaration in 1993.
Acknowledging Britain's attempt to recognise Irish Republicanism and achieve a compromise, the Cardinal said the IRA "would have nowhere to go, no political future, no place in the shaping of a future for Ireland, no hope of any access ever to any political dialogue or to any sharing of political power." if they refused to engage in the peace process.
His successor Cardinal Sean Brady visited Cardinal Daly in hospital on Monday.