Corbelled construction is a building technique in which each successive course of stone projects slightly further inward than the one below, creating a self-supporting arch or vault without the need for mortar. What makes this structure quietly arresting, beyond its age and its setting, is the material of those walls: purple slate, almost certainly ferried across the water from Valencia Island or Dolus Head, rather than quarried from the island itself. The walls average 1.4 metres at the base, faced on both sides with roughly dressed slabs and packed with a core of slate chippings, a method that speaks to considerable organisational effort for what is, internally, a modest space of 5.72 metres by 3.77 metres.

The oratory sits partly over the footprint of an earlier rectangular wooden building, suggesting a sequence of use on this spot before the stone structure was raised. Excavation revealed that the original floor was trampled earth layered with organic material, most likely straw or grass. The east gable carries a small window with inclining jambs and a stepped sill; a second window opens in the south wall at offset level. The west gable held a trabeate doorway, that is, one with a flat horizontal lintel rather than an arch, though the head has since collapsed. Its threshold slabs remain in place, and excavation there uncovered a heel-stone alongside two slabs bearing semicircular notches, interpreted as fittings for securing the door frame. Perhaps the most evocative finds from the site are two gable finials, the stone ornaments that would have crowned the roof ridges. One, recovered from the seabed to the east of the island in 1936, is of green sandstone and heart-shaped, with projecting side-wings and a tenon at its base for mounting. Despite heavy weathering, it retains carved decoration: geometric lozenge patterns on both sides, and on one face a full-length human figure with raised forearms, flanked at the head by stylised animal heads formed by grooving. The second finial, found elsewhere on the island, is of slate, roughly shaped into a shallow Y-form, and is undecorated. The concrete capping visible today along the tops of the walls was applied by the Office of Public Works during conservation work in the early twentieth century, a pragmatic intervention that preserved what remained even as it altered the profile of the ruin.