That detail alone makes this place unusual, but what catches the eye before any history is explained is the masonry itself: walls built from enormous stone blocks, some of them roughly three metres long and weighing several tonnes, fitted together with a precision that prompted a late 19th-century observer to remark, with some bewilderment, that such work had been achieved at a time when "stonecutters were not procurable." This style is sometimes called cyclopean construction, a term borrowed from the ancient world to describe stonework so massive it was once attributed to giants.

The foundation at Ardagh is traditionally associated with St Patrick, and St Mel is recorded as its first bishop. Under the wave of church reform that reshaped Irish ecclesiastical organisation in the 12th century, Ardagh became the seat of a diocese, and this building served as its cathedral. By the early 17th century it appeared on a map of Ardagh barony as a small ruin beside a larger, still-roofed church; a late 17th-century account dismissed it as "a mightie small chapel." Excavations in 1967 showed that the stone structure had been built over an earlier timber church, possibly dating to the 8th century. What stands today, partially reconstructed, measures roughly 10.35 metres east to west and 7.7 metres north to south, with walls about 0.9 metres thick. The gable ends carry projecting antae, narrow stone extensions beyond the wall face that are a characteristic feature of early Irish church architecture. Aside from a flat-headed lintelled doorway in the west gable, the fabric is plain and undecorated, its age and scale doing all the communicating.

The church sits within a graveyard that remains in use, with a 19th-century Church of Ireland building about 30 metres to the northwest. The contrast between the two structures is quietly instructive: the Victorian building is taller, more finished, more obviously a church; the early medieval ruin beside it is lower, rougher, and considerably older, its great stones still holding their ground after more than a thousand years of weather and neglect.