John Cardinal D'Alton appointed him as an assistant priest in Clonfeacle parish, but after Ó Fiaich returned to full health he commenced post-graduate studies in University College Dublin (1948–1950), receiving an MA in early and medieval Irish history.

[4] Ó Fiaich was an academic and noted Irish language scholar, folklorist and historian in the Pontifical University in St Patrick's College, Maynooth, the National Seminary of Ireland.

[8] Following the relatively early death from cancer of William Cardinal Conway in April 1977, Monsignor Ó Fiaich was appointed Archbishop of Armagh by Pope Paul VI on 18 August 1977.

[10] The first major event in Ó Fiaich's cardinalate was the first ever papal visit to Ireland from 29 September to 1 October 1979 by Pope John Paul II.

His major speech calling on all the organisations that were prolonging The Troubles to end their activities was made in the Archdiocese of Armagh and was followed by a visit to the Marian Shrine at Knock, County Mayo.

His approach, including visits to republican prisoners in the Maze, triggered many complaints but Ó Fiaich was always adamant that he had pastoral responsibilities and that the strict work of politics especially in an era of Margaret Thatcher as well as Taoisigh such as Jack Lynch and Garret FitzGerald, was not his sphere.

He stated: When hunger striker Raymond McCreesh died, Ó Fiaich said: While the Cardinal showed deep concern for the treatment of prisoners, he was equally critical of those who used violence to further the cause of Irish nationalism.

[12] In 1983, the Reverend David Armstrong, a Presbyterian minister, was forced to leave Limavady due to threats that followed his wishing Father Kevin Mullan's Catholic congregation "Happy Christmas".

Ó Fiaich died of a heart attack on the evening of 8 May 1990 while leading the annual pilgrimage by the Archdiocese of Armagh to the Marian shrine of Lourdes in France.

[16] The Cardinal Tomás Ó Fiaich Memorial Library, a registered charity, was officially opened in Armagh 8 May 1999 by the Northern Ireland Secretary of State, Marjorie Mowlam.