But cairns, which are deliberate accumulations of stone built for purposes ranging from burial to commemoration to territorial demarcation, have a way of outlasting almost everything else. This one on Rossard is oblong in shape, running roughly eighteen and a half metres from northeast to southwest and just over seven metres across, rising to between one and a half and two metres in height. The stones themselves are substantial, each boulder around a metre and a half long, and the overall impression is of something built to be seen and to endure.

The most likely explanation for this cairn is that it served as a boundary marker between townlands, those ancient territorial divisions of the Irish landscape that predate the Norman period and in many cases preserve land boundaries of considerable antiquity. Placing a visible structure of this scale on high ground would have made practical sense, visible from a distance and unambiguous to anyone working the land below. Two further cairns lie roughly thirty metres to the southwest, suggesting this part of Black Rock Mountain carried some sustained significance as a place where boundaries were acknowledged and perhaps contested. Whether the three cairns form a related complex or simply reflect repeated use of the same elevated ground over time is not certain, but their proximity is notable.

The site sits just off the mountain summit, which means the cairn occupies one of those slightly odd positions, close to the highest point without quite claiming it, that often turns out to be deliberate rather than accidental in ancient landscape planning. The granite boulders composing it are large enough that construction would have required coordinated effort, which raises questions the available evidence does not fully answer about who organised that effort and when.