These are bullaun stones, a type found at early medieval ecclesiastical sites across Ireland, where a deliberate hollow, sometimes cup-shaped and often water-filled, was ground into the rock. Their exact purpose remains a matter of debate; theories range from liturgical use to more practical functions such as grinding or pounding. What makes the pair here quietly arresting is how closely matched they are: the two boulders are very similar in shape, and the depressions sit in corresponding positions on both stones, as though one was deliberately made to mirror the other.

The stones came to closer attention during excavations carried out in 1977 and extended in 1979. A four-metre square cutting around the first bullaun produced a notably varied collection of objects from its upper layers, including whetstones, knife-blades, nails, clay pipe fragments, burnt bone, mortar, slag, and chert chippings. The second bullaun was only uncovered during the 1979 extension of that same trench. Around both stones, the soil was heavily mottled with charcoal flecks and burnt material, suggesting sustained activity in the immediate area over a long period. Among the finds was a fragment of a stone pestle, which researchers have suggested may have been used within the hollows of the bullauns themselves, lending some weight to the idea that these stones served a grinding or processing function, though the full range of their use on a site as layered as Inis Cealtra is difficult to pin down.