Set into the outer face of the north wall of Faithlegg church's chancel, rather than displayed in any prominent or ceremonial way, is a large stone with a single smooth hollow worn or carved into its surface. This is a bullaun stone, a type of ancient stone bearing one or more circular basins whose precise original purpose remains a matter of quiet scholarly debate. They are found across Ireland, sometimes at early ecclesiastical sites, sometimes in isolation, and are variously associated with early Christian ritual use, cursing traditions, or simply with the practicalities of grinding. What makes the Coolbunnia example quietly odd is its incorporation into standing masonry, the stone serving as building material while retaining its basin, a hollow roughly 42 centimetres across and 10 centimetres deep, sunk into a slab measuring just over a metre in length.

The record for this stone traces back to the Reverend P. Power, who documented the ancient ruined churches of County Waterford in the Journal of the Royal Society of Antiquaries of Ireland in 1890 and 1891. Power noted the bullaun in its current position in the chancel wall of Faithlegg church, a medieval ecclesiastical site sitting on a gentle north-west-facing slope in Coolbunnia. Exactly when the stone was built into the wall, and whether it was already at the site or brought from elsewhere, is not recorded. The church itself is a separate structure with its own history, and the bullaun may predate it considerably, repurposed at some point by whoever was doing the building.