It is easy to overlook, partly because of its modest size and partly because it has been folded so naturally into the fabric of the shrine that it reads less as an archaeological object than as a piece of furniture. Yet it is a bullaun stone, one of a class of early medieval stones featuring deliberate cup-shaped depressions ground into their surface, and it has been there long enough that someone has begun leaving religious objects inside its bowl.

Archaeologist Tony Miller recorded the stone in February 2012 during a visit to the Mass Rock and Holy Well site. He described it as a small, sub-rectangular block of sandstone measuring 0.33 metres long, 0.25 metres wide, and 0.15 metres thick. The bowl, which is carved into one of the shorter sides rather than the flat face, measures 0.13 by 0.1 metres and reaches 0.08 metres in depth. Bullaun stones are found across Ireland, often in association with early ecclesiastical sites and holy wells, though their precise original function remains debated; theories range from liturgical use to grain processing to simple water collection for ritual purposes. What is unusual here is the stone's continued active role. Rather than being treated as a relic of the past, it functions as part of a living devotional site, with the bowl serving as a small receptacle for votive offerings.

The Mass Rock itself is a reminder of the Penal era in Irish history, when Catholic worship was suppressed under colonial legislation and outdoor gatherings around natural or improvised altars became a common form of religious practice. The presence of a bullaun stone at such a site is not unique in Ireland, but the way this one has been incorporated into ongoing use gives it a particular quiet continuity.