It is a bullaun stone, a type of worked or natural boulder bearing one or more cup-shaped depressions, and examples are found across Ireland in early ecclesiastical settings, sometimes associated with ritual use of water or with cursing traditions. This particular one is unusually small, measuring just 44 centimetres in length, and its shape is the thing that sets it apart: it closely resembles a saddle quern, the kind of curved grinding stone used for processing grain, with a shallow oval hollow worn or cut into its upper surface.

The stone's double identity is what makes it quietly interesting. A saddle quern is a domestic, workaday object, the sort associated with meal preparation rather than sacred practice. Yet here, the leading interpretation is that this stone may have served as a stoup, a basin for holy water, set into the wall of the adjacent Kilmacnevan Church. The depression is modest, roughly 22 centimetres across at the top and only 8 centimetres deep, but that would be sufficient for a wall-mounted font at a church entrance. Whether the stone was selected because its existing hollow made it convenient, or whether it was shaped deliberately for that liturgical purpose, is not recorded. The two possibilities sit alongside each other without resolution, which is rather typical of early medieval material culture in Ireland, where the sacred and the practical often occupied the same objects.