That hollow, an oval basin measuring roughly 46 centimetres by 35 centimetres, is what makes this a bullaun stone, a type of ancient carved or ground rock found at early Christian sites across Ireland. The precise function of bullauns is still debated; theories range from the practical, grain-grinding or pigment preparation, to the devotional, with many such stones accumulating folklore about healing water that collects in their basins.

Inis Cealtra, the island in Lough Derg known in English as Holy Island, was one of the more significant early Christian monastic sites in Munster, associated with Saint Caimin in the seventh century and bearing the remains of several churches, a round tower, and other features accumulated across centuries of use. This particular stone sits about 100 metres north of the structure known locally as the Confessional, tucked into the vegetation at the island's north-eastern edge. It is a substantial piece of rock, roughly 1.1 metres measured north to south, though ivy has made a full examination difficult. R. A. S. Macalister recorded it in his work from 1916 to 1917, and it was noted again by McNamara in 1984, which means it has been known to scholars for over a century without becoming any easier to reach or observe.

The stone's location at the far corner of the island, in genuinely overgrown ground, means a visitor is unlikely to come across it by accident. It lies well away from the main cluster of ecclesiastical remains that draw most attention on the island, and the ivy covering means the basin itself may not be immediately visible without a close look.