The church served the parish of Ishartmon, a name that the nineteenth-century scholar John O'Donovan traced, writing around 1840, to the Irish Diseart Munna, meaning St. Munna's church or hermitage. By the time of Oliver Cromwell's Down Survey in 1655 to 1656, a terrier, or written commentary accompanying the survey maps, described the building as decayed, and the parish had probably already collapsed as an active unit, since it goes unmentioned in the Visitation of Bishop Ram in 1615.

What survives is more complete than the word "ruin" might suggest. The nave and chancel both stand to their full height, apart from the north nave wall, and the west gable is ivy-covered but intact, carrying a double bellcote above it. A narrow pointed doorway, just over eighty centimetres wide and cut from undressed stone, opens near the west end of the south nave wall. The chancel arch, originally rounded and roughly a metre and a third wide, was later reworked into a pointed shape using brickwork, a small intervention that layers two distinct building periods into a single opening. The nave walls project outward beyond the chancel walls by about 1.2 metres, a deliberate arrangement that allowed the interior of the chancel to be generously proportioned while keeping the overall external ratio of nave to chancel at roughly two to one. Inside the chancel, small pointed windows survive in both the north and south walls, each retaining glazing grooves, and a blocked aumbry niche, a small recess used to store liturgical vessels, sits at the east end of the south wall. A projecting slab on the north side of the chancel interior may once have supported a statue. The font in the nave, circular and cut from granite, measures just over sixty centimetres in external diameter, and its continued presence in the building is something of a puzzle given how long the parish has been out of use.