Known also as St. Molaise's Chapel, it sits close to the inner face of the Cashel wall, the great dry-stone enclosure that rings the island's monastic settlement off the Sligo coast. Most early Irish churches face roughly east-west as liturgical custom demands. This one does not. Its orientation of 64°30' puts it noticeably out of step, and no entirely satisfying explanation for that divergence has been settled upon.

The building's peculiarities are not accidental; they are the accumulated result of substantial rebuilding over a long period. The structure as it now stands is trapezoidal in plan, widening slightly towards the east, with an asymmetrical stone roof of very low pitch, grass-covered, raised in the late medieval or post-medieval period on timber formwork whose lath impressions are still visible in the mortar inside. The original building, reconstructed from surviving fragments of earlier masonry in the gable walls, was narrower and almost certainly taller, probably carrying a steeply pitched wooden roof clad with thatch or shingles. When the side walls were rebuilt, the structure was widened by roughly 0.6 metres, changing its proportions from something close to 1.4:1 to something closer to a square. A charcoal-bearing mortar sample from the east wall returned a radiocarbon date centring on AD 724 to 836, though as that wall was itself substantially rebuilt, probably in the later medieval period, the mortar may not have been in its original position when sampled. Inside, the chapel is almost entirely filled by stone furnishings: a high narrow altar bench spanning the east wall, and along the south wall a broader, lower bench known as Leaba Molaise, meaning St. Molaise's bed. The doorway lintel carries a small incised Greek cross, and the east window retains traces of a sophisticated pivoting shutter system, possibly originally lined with an inner frame of stretched hide to admit light while keeping out the Atlantic weather.

The chapel was traditionally identified as the tomb or shrine chapel of St. Molaise, patron saint of the island, and housed his secondary relics until modern times. It is now closed by a padlocked metal gate set into the doorway, which allows a view of the interior without entry. The building is generally sound, and the contrast between the well-fitted squared rubble of the surviving original masonry in the gable ends and the looser, more irregular stonework of the rebuilt side walls is legible to anyone who looks closely at the fabric of the walls.