The second of the cathedrals of Armagh, the Roman Catholic Cathedral is sited on a hill to compete with the older Church of Ireland Cathedral. This cathedral has a chequered history with many architects involved. The original building was commenced in 1840 by Thomas Duff of Newry in a 16th century gothic idiom. This was stopped due to the Famine. It was started again in 1850 by J.J. McCarthy is his favoured 14th century gothic and completed around 1870. The change in architectural style can be seen on the main front where the entrane doorways and aisle windows are at odds with the design of the rest of the building.
The exterior of the cathedral is quite austere. Despite the twin spires on the main front, the exterior has none of the decoration associated with other cathedrals being built in Ireland at this time. The main external decoration is an arcade of statues over the main doorway as well as a large window. The interior has been altered over the years with Vatican II re-ordering removing much of the original elaborate altars and fittings
The original interior with its mosaics, marble, gilt and fittings was designed by G.C. Ashlin and was mostly removed by Liam McCormack during his post Vatican II renovations in 1977-82. Like at nearby St. Macartans in Monaghan, this reorganisation of the sanctuary ruined the interior of the building. In 2003 the cathedral reopened after a major restoration. This restoration removed many of the mistakes of the previous reordering. In particular the altar area has lost the intrusive tabernacle and altar (Liam McCormack).