A large, irregular depression, measuring roughly 0.43 by 0.53 metres, has been worn or worked into the stone, and within that hollow are three smaller circular depressions, each between 0.13 and 0.21 metres across and up to 0.14 metres deep. This is a bullaun stone, a type of ancient carved rock found at early Christian sites across Ireland, typically characterised by these cup-shaped basins ground into their surface. Their exact function remains debated: they may have served liturgical purposes, been used for grinding or mixing, or accumulated a ritual significance that accumulated over centuries of use. Whatever its original purpose, the stone at Cill Chuáin is a compact and quietly striking example, its nested depressions lending it an almost deliberate, considered quality.

The site around it has its own interrupted history. Documentary evidence suggests a church existed here by 1475, when it appears in the Calendar of Papal Registers. By 1622, however, it had dropped out of the diocesan record entirely, absent from a list of churches compiled in a Trinity College Dublin manuscript. The implication is that the building had already fallen into disrepair sometime in the intervening century and a half. The graveyard continued in use, as such places often do long after their associated churches have gone, and the modern rectangular enclosure that occupies the site today sits directly over or beside whatever remained of that medieval parish church. The bullaun stone predates any of this written history, belonging to a layer of the site that the documents cannot fully reach.