A flat slab of stone, roughly 1.6 metres across and raised on smaller supporting stones to the height of an altar, sits pressed against the innermost face of the circle's most southerly orthostat, one of the large upright stones that form the ring. The arrangement is deliberate and careful, and one of the stones propping up the slab appears to be a fallen orthostat from the circle itself, meaning the builders of the makeshift altar were working with whatever the ancient monument had already provided.

The slab is a mass rock, a term for the improvised outdoor altars used by Catholic priests in Ireland during the Penal Law era, roughly the late seventeenth and early eighteenth centuries, when Catholic worship was suppressed under legislation that banned the formal practice of the faith. Congregations would gather at remote, often elevated spots, sometimes with a lookout posted, to attend Mass said over a flat stone serving as an altar. That someone chose the interior of the Derrynafinchin stone circle for this purpose adds a particular texture to the site. The circle itself is a Bronze Age monument, thousands of years older than the congregation that sheltered within it, and the choice may have been practical, offering a degree of enclosure and concealment, or it may have carried some older, unrecorded sense of the place as one set apart for ceremony. A local history of the Borlin Valley, "Hidden Gold: History and Folklore of the Coomhola and Borlin Valleys", edited by Julia Kemp and published in 1998, records the tradition of a mass rock at this location, suggesting the memory of its use was preserved in the community long after the Penal Laws had passed.