The stone, which measures less than a metre across, contains a single oval depression roughly the size of a large fist, worn some sixty millimetres into the surface. Whether it is a bullaun or a cup-marked stone, two related but distinct categories of prehistoric or early medieval rock art, is not entirely settled. Bullauns are rounded, bowl-like hollows found most often near ecclesiastical sites and holy wells, and were traditionally used for grinding or associated with ritual; cup marks, by contrast, are shallower and more often found in prehistoric contexts without any obvious religious setting. At Grousemount, the hollow sits somewhere between the two definitions.

The stone came to light in 2016, when the archaeological consultancy John Cronin and Associates carried out fieldwork ahead of a wind farm development by ESB Wind Development Ltd in the Grousemount area of Kilgarvan. The survey, conducted under licence, recorded the rock sitting on a moderate downward slope, a small and solitary find. A search of the immediate surrounding area turned up no further archaeological features, which makes this hollow all the more quietly puzzling. There is no associated monument, no cluster of related stones, nothing to place it within a familiar pattern. It simply sits there, a single deliberate mark on a piece of bedrock, its purpose and age unresolved.