Known locally as the Mass Altar, it measures roughly three metres by one and a half, its upper surface lying level with the surrounding ground despite the incline beneath it. That levelness is deliberate: towards either end of the southern side, two upright supporting stones, known as orthostats, hold the slab in place, and between them a shallow chamber has been dug out from below, dropping around sixty centimetres into the earth.

The site belongs to the tradition of mass rocks, outdoor locations where Catholic priests celebrated the sacraments during the Penal Law era, roughly the late seventeenth and early eighteenth centuries, when Catholic worship was prohibited or severely restricted under British legislation. Congregations would gather at remote or inconspicuous spots, often on hillsides or in hollows, where a flat rock served in place of an altar. The Coolmountain example is unusually structured: the sub-slab chamber and the orthostatic supports give it something closer to a constructed monument than a simple opportunistic outcrop, suggesting either deliberate adaptation of an existing feature or, perhaps, a longer local memory attached to the stone itself.