This is a bullaun stone, a type of ancient rock monument found across Ireland in which one or more rounded basins have been ground or worn into the face of a boulder. Their precise origins and purposes remain debated; they are associated variously with early Christian sites, pre-Christian ritual practice, and practical uses such as grinding. What makes the Brockagh example worth noting is its layout: two proper basins sit at opposite corners of the rectangular stone, with a shallower, slighter depression between them, giving it a deliberately articulated quality that is hard to attribute to accident.

The stone was identified and measured by Healy in 1972, who recorded the boulder as approximately 1.8 metres by 2 metres, with the bulk of it remaining below ground. The two main basins differ slightly in dimension; one measures roughly 34 centimetres by 33 centimetres and reaches about 15 centimetres in depth, while the other is 38 centimetres by 31 centimetres and 14 centimetres deep. The intermediate depression is considerably shallower, only 3 centimetres deep and around 30 centimetres across, more of a worn hollow than a true basin. Whether that central feature is incidental or part of the original design is not recorded, but its presence between the two main basins gives the stone an unusual, almost structured character.