The name comes from the Ordnance Survey six-inch map, but whether anyone ever actually made confession here is genuinely uncertain. The building measures just 3.1 metres east to west and 2.45 metres north to south, and much of that interior is taken up by four inclined standing stones arranged around what excavations in the early 1970s revealed to be a shallow cist-like construction of limestone flags, a cist being a box-like structure formed from flat slabs, typically used to contain human remains or precious objects. That detail alone shifts the building's probable purpose away from penitential ritual and towards something older and more ambiguous.

Excavation showed that the stone structure visible today dates in its present form to around AD 1700, but it had been rebuilt several times before that. Earlier still, it appears to have replaced a timber structure or shrine slightly to the west, which had itself gone through multiple phases of rebuilding. The leading interpretation is that the building functioned as a reliquary, a structure housing sacred relics, though this has not been confirmed beyond doubt. Adding to the layered chronology, a recumbent cross-slab aligned with the building to the east, and featuring a slot for an upright slab, has been dated on stylistic grounds to the tenth or eleventh century. The whole complex sits on a narrow east-facing shelf just north-east of the Saint's Graveyard, and once stood within a circular enclosure. Inis Cealtra was an important early Christian monastic site, and the round tower and church of St Caimin visible nearby serve as a reminder that this modest little structure belongs to a landscape dense with centuries of religious activity.