A bullaun stone is a naturally occurring or dressed rock into which one or more rounded hollows have been deliberately ground, most commonly associated with early Christian monastic sites and sometimes linked to ritual or curative practice. This one, the largest of several partly submerged rocks in the area, measures roughly 1.5 metres by 0.85 metres and stands about 0.8 metres high, subrectangular in plan with a noticeably straight north-eastern side. On its south-western face sits a single circular basin, around 37 centimetres across, deeper on its eastern edge than its western one, the whole top surface described as very uneven.

The stone's current position in the water is not its original one, at least not in any meaningful sense. When the Ardnacrusha hydroelectric scheme was developed on the River Shannon in the 1920s, weirs constructed as part of the project raised the level of the lake and flooded stretches of the shoreline. Inis Cealtra, known in English as Holy Island, was already a site of considerable early medieval significance, associated with Saint Caimin and home to a cluster of churches, a round tower, and early grave slabs. The rising water transformed the island's margins, pushing what had been shoreline features out into the lake. The bullaun, documented by Macalister in 1916 to 1917 and again by McNamara in 1984, was presumably accessible on or near dry land before the scheme altered the hydrology of the area permanently.