This is a bullaun stone, a type of ancient rock feature found across Ireland, typically characterised by one or more cup-shaped depressions ground into the surface, and long associated in folk tradition with healing, cursing, and sacred water. The Aherla example was a substantial piece: roughly five feet by four feet across the top and two feet in height, with a smooth, level upper surface and a single oval hollow at its centre, measuring fourteen inches by eleven inches and nine inches deep.

When the researcher Hartnett recorded the stone in 1939, he noted not just its physical dimensions but the layered tradition surrounding it. Local people believed it had curative powers and preserved a memory that mass had been celebrated at the site during the Penal Laws, the period from the late seventeenth century onward when Catholic worship in Ireland was suppressed by statute and priests conducted services in remote, concealed locations. In that context, a large flat-topped boulder in a secluded glen would have served well as an improvised altar, and the overlap between pre-Christian sacred stones and the clandestine Catholic practice of the Penal era is a recurring pattern across the Irish landscape. Dr D.D.C. Pochin Mould photographed the stone in 1981, which makes her record particularly poignant: at some point after that photograph was taken, the stone was removed. Where it went is not recorded, and the glen near Aherla now holds only the tradition where the object once stood.