The structure is a wedge tomb, a type of megalithic burial monument built roughly between 2500 and 2000 BC, typically characterised by a long stone gallery that is wider and taller at the western entrance end and narrows progressively toward the east. But in Knocknagappul, the locals have long called it the mass-rock, a name that speaks to a different and much more recent layer of history: the Penal Law era, when outdoor flat stones served as improvised altars for Catholic worship conducted in secret. Whether the community genuinely gathered here for clandestine Masses or simply recognised the flat slab as a plausible setting for such a gathering, the name has stuck, folding two very different pasts into the same stone.

The tomb itself is aligned WNW-ESE, as is typical for wedge tombs, with its wider entrance opening toward the west. The gallery runs at least four metres in length, measuring 1.3 metres wide at its western end and narrowing to around 0.8 metres toward the centre. Two jamb stones set lengthways just inside the western entrance reduce that opening further still, to 0.65 metres. Four stones define the southern wall of the gallery; only two remain on the northern side, and both the eastern end stones and the backstone are gone. A single large roofstone, measuring roughly 2.2 by 2.3 metres and tapering slightly in thickness from west to east, covers the eastern section of the gallery. Seven outer-wall stones curve around the eastern end of the structure, forming a kind of kerb. A prostrate slab lying just south-east of this outer walling, measuring 1.7 metres by 0.55 metres, may be a displaced second roofstone. A second wedge tomb lies approximately 300 metres to the east, making this part of Museramore Mountain unusually dense with prehistoric funerary remains.