It is a bullaun stone, a type of early medieval stone with one or more cup-shaped hollows ground or worn into its surface, and examples like it are found across Ireland at monastic and sacred sites. What makes this one quietly peculiar is its position: rather than standing freely in a churchyard or beside a holy well as many bullauns do, it has been built directly into the fabric of a cathedral wall, incorporated into the internal face of the stonework near the north-east angle.

The stone was recorded by Healy in 1972 as a boulder measuring roughly 0.58 metres by 0.58 metres, with a single basin of about 0.28 metres in diameter and 0.09 metres in depth. Those are modest dimensions, the basin little wider than a spread hand, but the deliberateness of the hollow is unmistakable. How or when it came to be set into the wall is not recorded, and whether it was placed there intentionally as a kind of re-use of an older sacred object, or simply pressed into service as convenient building material, remains an open question. Either possibility says something interesting about how early Christian communities in Ireland related to objects that predated or accompanied their own traditions.