It measures roughly three metres long and a metre wide, unremarkable to the casual eye, yet locally it carries a specific and weighted name: a mass-rock. That name alone signals a particular chapter of Irish history, one played out on hillsides, in bogs, and at cliff faces across the country during the Penal Law era, when the public practice of Catholicism was suppressed under legislation introduced in the late seventeenth and early eighteenth centuries. Priests who continued to celebrate Mass did so in secret, outdoors, often using a flat stone or natural ledge as an improvised altar. The congregation would gather in remote places where approach could be watched and discovery avoided.

The rock at Glanatnaw fits this pattern closely. It occupies a sheltered position at the base of the cliff, set into the hillside rather than exposed on open ground, and faces away from the most travelled directions. A narrow ledge on its south-east side still provides the only real access to its upper surface, which suggests the stone was used deliberately rather than incidentally. Whether any specific date or priest is attached to this particular site has not been recorded, but the landscape itself, high pasture rough enough to deter casual visitors, with a stand of coniferous trees now growing immediately to the north-east, gives some sense of the isolation that made such spots useful.

The rock is not easy to spot. The heather that covers much of the surface makes it blend into the surrounding hillside, and finding the narrow access ledge on the south-east side requires knowing what to look for. The site sits to the north-east of the hill's summit, so approaching from that direction and moving down toward the cliff base is the most direct route.