The building is narrow and long, just under eighteen metres east to west and only four and a half metres wide internally, with no internal division between nave and chancel. What makes it quietly unusual is how much survives despite centuries of neglect: glazing bar-holes still visible in the window stone, an aumbry, a small wall recess used to store sacred vessels, at the east end of the south wall, and even a cupboard space internally beside the destroyed east window.

The church's history is one of institutional ownership, gradual decline, and eventual abandonment. By the early fourteenth century it was listed among the possessions of Mellifont Abbey, the great Cistercian house in County Louth, appearing in the ecclesiastical taxation carried out between 1302 and 1306 under Pope Nicholas IV. When Mellifont was suppressed in 1540, the abbey still held over 150 acres in the area, then known as the Newton of Knokamothan, and had been providing the local chaplain, one Peter Rus. By 1622, the bishop and scholar James Ussher noted the church as only indifferently repaired, though the chancel had seen some attention. The real break came in 1641, after which, according to two later episcopal visitations in the 1680s and 1690s, the church of the Blessed Virgin Mary at Knockcommon had fallen entirely out of repair. The walls were still standing at that point, but the graveyard itself was not even enclosed. It has the feel of a place that slipped quietly out of use rather than suffered any single dramatic event.