It is a mass-rock, a term for the outdoor altars used by Catholic priests to celebrate Mass during the Penal Law era, when Catholic worship was suppressed and clergy risked arrest, transportation, or worse. Congregations gathered in remote spots, uplands and hollows away from roads and settlements, where they could not easily be observed or reported. The rock itself served as the altar.

The site sits just below the summit of Knockseefin, immediately north of the townland boundary with Gortnanuv, and roughly a hundred metres south-west of a burial mound that occupies the very top of the hill. By 1840, when the Ordnance Survey produced its first detailed six-inch maps of Ireland, the place was already recorded under the name Carriganaffrin. Fifty years later, the 1897 edition of the more detailed twenty-five-inch Ordnance Survey maps depicted it differently, showing the cliff-edge face of an old quarry rather than marking any religious association. The landscape had been read and re-read by cartographers across two centuries, each edition catching a different layer of use and meaning at the same outcrop.

The rock is visible on satellite imagery from late 2018, though scrub vegetation has begun to encroach on it and may have thickened further since. Anyone making for the site should be prepared for rough, uneven ground on the ascent to Knockseefin, with no formal path or waymarking indicated in the records. The burial mound at the summit acts as a rough landmark; the mass-rock lies to its south-west, just short of the top. Early autumn, when vegetation has not yet fully died back but sight lines across the hill are still reasonably clear, tends to be a practical time for finding features like this in upland scrub.