What draws closer attention is the east wall, which has survived in notably good condition relative to the rest of the structure, its tall round-headed window still intact within a semicircular-arched embrasure, the stonework splayed inward in the way medieval builders used to draw light deep into narrow interiors. The well-defined stops at the base of the embrasure suggest the sill once sloped upward, though the stone there has largely been robbed out over the centuries, a common fate for dressed masonry in rural areas where building material was always in demand.

The church is reputedly the foundation of St Onchon, also known as Onchu, identified by the antiquarian Thomas Johnson Westropp in 1900 as probably the son of one Blathmac. Onchon is said to be buried alongside Finian at Clonmacnoise, the great monastic site on the Shannon, and his feast day falls on the 9th of July. The structure that carries his name is a modest one by medieval standards, measuring roughly 15.6 metres east to west and 6.5 metres north to south, built of large limestone blocks set on a plinth. The corners are constructed in side-alternate quoining, a technique where stones are laid alternately upright and flat to bind the corners together, and the walls sit on a slight outward lean, or batter, particularly visible on the northern exterior. The south wall has been substantially rebuilt in recent times, which gives the building an uneven quality, parts ancient and parts considerably less so. Six grave markers remain inside, three headstones in the western half and three flat slabs in the east, with the earliest legible inscriptions dating to the 1890s. The site was recorded on the Ordnance Survey six-inch maps of both 1842 and 1915, suggesting it was already well established as a named ruin by the mid-nineteenth century.