It is not large, measuring no more than 0.6 metres across and roughly 0.2 metres thick, and it would be easy to walk past without a second glance. What sets it apart is a single circular depression worn into its upper surface, 0.28 metres across and 0.09 metres deep, the defining feature of what is known as a bullaun stone.

Bullaun stones are found at early Christian and pre-Christian sites across Ireland. The word comes from the Irish "bullán", meaning a rounded hollow, and the stones are essentially boulders or slabs bearing one or more such cup-shaped basins, ground smooth over centuries. Their precise original purpose remains unclear; they have been interpreted variously as mortars for grinding pigments or grain, as water-collecting vessels associated with ritual or healing, and as markers of sacred space. The stone at Coolnaha sits within the eastern half of a broader ecclesiastical enclosure, the kind of roughly circular boundary, often surviving only as a slight rise in the ground or a curving field boundary, that typically surrounded an early Irish church and its associated land. That this particular bullaun occupies a corner of a cillin, a children's burial ground where unbaptised infants were traditionally interred outside consecrated ground, adds another layer of meaning to a site already dense with the long continuity of Irish religious practice.