It measures just 1.7 metres long, 0.6 metres wide, and just over a metre high, modest enough to be mistaken for a field boundary feature or a forgotten agricultural remnant. It is, according to local tradition, a mass altar, one of the makeshift sacred spaces that quietly proliferated across the Irish countryside during the Penal Laws era.

The Penal Laws, a series of statutes enacted from the late seventeenth century onward, severely restricted Catholic worship in Ireland, prohibiting priests from officiating openly and driving religious practice into fields, hillsides, and out-of-the-way enclosures. The so-called mass rock became a practical response to this, a flat or fashioned surface in an isolated location where a priest could celebrate the Eucharist with a congregation gathered informally and out of sight. What makes the Cahergal site particularly vivid is the detail preserved alongside it: to the north-east of the enclosure stands a large natural boulder, identified in local memory as the lookout post used while mass was being celebrated. Someone, presumably a trusted member of the community, would have watched from that elevated vantage point for approaching authorities, ready to raise the alarm. The two features together, altar and lookout, give a rare sense of the practical, nervous organisation behind what was, under the law of the time, a criminal act of worship.