It is a bullaun stone, or rather it appears to be one; the mystery lies in what it is actually cut into. Bullauns are bowl-shaped hollows ground into rock or stone, found across Ireland at early Christian and prehistoric sites, and traditionally associated with ritual use, healing, or the grinding of grain or pigment. This one, however, has an unusual quality that sets it apart from the typical fieldstone example.

When Tony Miller visited the site and reported the stone in January 2013, he observed that the bullaun is not carved into a natural boulder but into a circular piece of masonry, roughly 0.75 metres in diameter and with a regular thickness of around 0.18 metres. The shape and regularity of the masonry raised an immediate question: is this fragment part of the wider collection of stonework at the Kilnaruane site, all of it connected to the early Christian complex, or could it be something more prosaic and more practical, a broken millstone, repurposed or simply abandoned in place? Miller noted that only excavation could resolve the matter definitively. The bullaun sits approximately six metres north-east of the cross shaft, itself a significant piece of early medieval carving, and the ground beneath it is the central burial area of the enclosure. That layering of possible meanings, ritual hollow, grinding stone, broken mill fragment, accumulated over the centuries on a site already dense with early Christian remains, gives the stone an ambiguity that no amount of surface inspection alone can settle.