This is a cairn, a monument built from heaped stone rather than earth, and the scale of it is not negligible: roughly fifteen metres across east to west, fourteen metres north to south, and rising to about two and a half metres in height. That combination of a levelled top and sharply angled flanks is characteristic of prehistoric funerary cairns, which were typically raised over burials during the Neolithic or Bronze Age periods, though without excavation it is impossible to say with certainty what lies beneath or who built it.

Cairns of this type are found across Ireland, often placed at or near hilltops in ways that suggest their builders were concerned with visibility and landscape, both seeing and being seen across considerable distances. Knocknaskagh is not a dramatically prominent peak, but a summit location still carries that quality of deliberate placement, a decision made by people who wanted the monument to occupy a specific point in the terrain. The forestry that now surrounds the cairn has changed the experience of approaching it considerably; what was once an open hillside monument exists today in a more enclosed, shadowed setting, the trees pressing in around something that was almost certainly meant to be seen from afar.