It measures less than a metre across in either direction, barely enough for a priest to set down the vessels required for Mass. Yet that, according to local tradition, is precisely what it was used for, a natural shelf shaped by the land itself into something that functioned, at need, as an altar.

Mass-rocks belong to the era of the Penal Laws, the series of statutes enacted from the late seventeenth century onwards that severely restricted Catholic worship in Ireland. With churches suppressed and clergy subject to arrest or worse, congregations gathered outdoors at agreed locations, often in remote or sheltered spots that offered concealment and a quick means of dispersal. A flat-topped stone or a natural ledge of bedrock would serve as the altar. The Knockbrandon example is modest even by those standards, the "arm-chair arrangement" of stones creating a shallow shelf rather than a broad table, suggesting a site chosen for its seclusion as much as its convenience. The valley's steep sides and north-facing aspect would have kept it in shadow and out of easy sight, which was rather the point. The site appears on the 1940 edition of the Ordnance Survey six-inch map, meaning its identity as a Mass-rock was still part of living local memory at that date, passed down through the community long after the conditions that created it had passed.