The traditional founding story begins with a pagan chieftain named Daire who refused to let Saint Patrick build a church on Ard Mhacha, the prominent hill near his fort. Daire instead granted Patrick lower ground to the east. Some time later, Daire's horses died after grazing on the church land, and he ordered his men to kill Patrick. He himself fell ill before they could act. His servants begged Patrick to heal him; Patrick sent holy water that revived both Daire and the horses. In gratitude, Daire gave Patrick a great bronze cauldron and finally the hill itself. Patrick built a stone church on the summit and a monastery beside it. The school he founded soon became famous and was said at its peak to hold seven thousand students. The story may be ninety per cent legend. The hill is still there. So is the Church of Ireland cathedral built on the site Patrick chose.

Around the year 807, a scribe named Ferdomnach completed in the Armagh scriptorium an extraordinary illuminated manuscript that contained the only complete New Testament copied in Ireland during the early medieval period, along with three accounts of Patrick's life and his Confessio. It is now called the Book of Armagh, also known as the Canon of Patrick. The manuscript was the property of the archbishops of Armagh for over six centuries, and was carried with them on their official circuits through Ireland as evidence of their authority. The Book of Armagh contains a note in the handwriting of Brian Boru's own chaplain confirming that the High King had granted his sanction to the Law of Saint Patrick in the year 1006. Brian was killed at Clontarf in 1014 and buried at Armagh. The Book itself is now held in the library of Trinity College Dublin.

Of all the archbishops in sixteen centuries of succession, the most famous is Saint Oliver Plunkett, primate from 1669 to 1681. He served during the worst years of the penal laws, and spent much of his episcopate in hiding and on horseback, conducting confirmations in the open fields. Plunkett was caught up in the wave of anti-Catholic hysteria of the Popish Plot. He was tried in Dundalk where the Protestant jury refused to convict him. The case was moved to London, where he was tried without his witnesses and condemned. On 1 July 1681 he was dragged on a sledge to Tyburn and hanged, drawn, and quartered before a vast crowd. He was the last Catholic to be executed in England for religious reasons. His head, preserved with remarkable freshness, can still be viewed at Saint Peter's Catholic Church in Drogheda, in the same archdiocese he led. He was canonised by Pope Paul VI in 1975.

From the 12th century onward, the Archbishops of Armagh have been engaged in a long and frequently bitter argument with the Archbishops of Dublin over which see holds primacy. Both archbishops are called "primates": Armagh is Primate of All Ireland, Dublin is Primate of Ireland. The distinction matters in the protocol of the universal church. The argument turned formal in 1672 when Archbishop Plunkett published a treatise titled Jus Primatiale defending Armagh's ancient prerogative. The Dublin archbishop, Peter Talbot, replied two years later with Primatus Dublinensis. Two papal briefs from Pope Clement XI in 1719 came down in favour of Armagh, but neither archbishop in practice exercises supervisory authority over the other today. The dispute has long since become ceremonial. It is, however, still observed when both prelates appear at the same ceremony. Armagh comes first.

The current Cathedral of Saint Patrick is the second on the site. Begun in 1838 by Archbishop William Crolly, the first Catholic primate to reside in Armagh since the Penal Laws were enforced, the cathedral took more than sixty years to complete. The foundation stone was laid on 17 March 1840. Construction was paused by the Great Famine of the 1840s and resumed afterward. Cardinal Michael Logue, primate from 1887 to 1924, spent more than £50,000 of his own and the church's money completing and decorating the building. It was consecrated on 24 July 1904 in the presence of Cardinal Vincenzo Vannutelli, representing Pope Pius X. Twin spires rise from its eastern front. From the city below, it stands directly opposite the Church of Ireland Cathedral of Saint Patrick, on a parallel hilltop, also dedicated to the same saint.

Eamon Martin has been Archbishop of Armagh since 2014. He was appointed coadjutor in 2013, succeeded Cardinal Seán Brady upon his retirement, and has led the Irish Catholic Church through a difficult period defined by clerical abuse revelations, falling Mass attendance, and the rapid secularisation of Irish society. He is assisted by Auxiliary Bishop Michael Router, appointed in 2019. The archdiocese still includes eight suffragan dioceses: Ardagh and Clonmacnoise, Clogher, Derry, Down and Connor, Dromore, Kilmore, Meath, and Raphoe. Its territory spans Northern Ireland and the Republic, and includes the cities of Armagh, Drogheda, and Dundalk, and the large towns of Coalisland, Cookstown, Dungannon, Magherafelt, and Ardee. In 2016, the Redemptoris Mater Seminary opened in Dundalk under the Neocatechumenal Way; its first ordination was in 2014. The work continues. The hill of Ard Mhacha continues to be the centre of something quite ancient and quite alive.