There are no dressed stones, no carved doorways, no upstanding walls. The rectangular outline of the church, roughly twenty metres east to west and just over nine metres north to south, survives only as a slight rise in the ground, its western end trailing off into a low cairn. Around it, an oval graveyard enclosed by a stone-faced scarp and mature deciduous trees holds a small number of headstones, most dating from between about 1790 and 1820, suggesting the site had largely fallen out of use long before those stones were cut.

The history of Mullandoy reaches back considerably further. The site is traditionally associated with St Maoldóid, a seventh-century saint descended from the Uí Mhéith, the early Irish kindred from whom the barony of Omeath in Louth takes its name. Whether or not he founded a monastery here cannot now be confirmed, but whatever community existed at Mullandoy was substantial enough to attract unwanted attention: it was plundered by Vikings in 830, and a succession of coarbs, the hereditary successors to a monastery's founding saint, are recorded at the site from 956 to 1161. By the early fourteenth century the church appeared in the ecclesiastical taxation of Pope Nicholas IV, listed between 1302 and 1306 as de Mocynam. It remained the parish church of Muckno for several more centuries, though by the time of Archbishop Spottiswood's visitation in 1622, the administrative centre of the parish had already shifted to Castleblayney, even if a replacement building had not yet been erected there. Catholic Mass was still being said at Mullandoy at that point. The formal break came in 1690, when Lord Blayney built a chapel near his castle at Ballynalurgan, which then became the parish church of Castleblayney. One object from Mullandoy survived the long decline: part of an octagonal font shaft, decorated with figures and dated to the fifteenth or sixteenth century, was removed from the graveyard and is now held by the Monaghan County Museum.