It is a bullaun, a type of rock, often a large boulder or slab, into which one or more cup-shaped depressions have been deliberately ground. Nobody agrees entirely on what they were for. Ritual use, grain grinding, and the collecting of water considered to have curative properties have all been suggested, and in many cases the answer is probably all three at different points across the centuries.

This particular example was recorded by Healy in 1972, who noted its modest dimensions: roughly 0.77 metres long, 0.6 metres wide, and 0.23 metres high. It carries a single basin, approximately 0.29 metres by 0.31 metres across and 0.14 metres deep, with the floor of the hollow sitting slightly off centre, a small irregularity that speaks to the handwork involved in its making. Harold Leask, whose guidebook to Glendalough appeared in 1950, included it as item thirty-eight in his catalogue of the site's features, suggesting it was considered significant enough to document even alongside the grander stonework nearby. By 2005 it had made its way into the visitor centre exhibition, which at least ensured it received some attention beyond specialist literature.