The stone is a prostrate slab of shale, roughly 2.7 metres long and 2 metres wide, broken at its southern end, with pock-marks visible on its south-eastern corner. Beside it stands a smaller upright sandstone pillar, oriented east to west. Locally, the place is known simply as the mass rock, a name that points to a specific and sombre chapter of Irish religious life.

Mass rocks are sites where Catholic priests celebrated the Eucharist outdoors and in secret during the Penal Law era, broadly the late seventeenth and eighteenth centuries, when Catholic worship was suppressed under legislation that banned priests from operating openly and forbade the building of Catholic churches. Congregations would gather in remote fields, on hillsides, or in hollow ground, using a convenient flat stone as an improvised altar. The risk was real: priests caught saying Mass could face transportation or execution, and those who sheltered them faced serious penalties. The Killinga stone, a broad shale slab lying on level ground, would have served precisely this purpose, with the adjacent upright pillar perhaps marking the spot or serving some practical function during the ceremony. The pock-marks on the stone's south-eastern end are intriguing; such marks sometimes result from repeated use or from the fixing of a cross or other object, though no specific explanation for these particular marks has been recorded.