Shallow pits in the surface of the earlier cairn, on its south-western to northern arc where the stonework is loosest and least defined, appear to mark exactly where material was lifted and reused. The older cairn, roughly circular and about twelve metres across, is flat-topped and composed of compacted, lichen-covered stone; the newer one rises nearly three metres above it in a rough cone of dry-laid slabs, its lower courses set horizontally with some care, its upper portions simply stones and slabs wedged into place.

The modern structure almost certainly carries the name 'Mackenzies Monument', a label associated with the maritime surveying work of Murdoch Mackenzie in 1774. Mackenzie was a pioneering hydrographer whose meticulous charting of Irish and British coastal waters set new standards for navigational cartography, and a prominently placed marker on a ridge visible from many points on the island and across the surrounding sea would have served as a useful triangulation point for that kind of work. The position is well chosen: although the true summit of Knockmore, at 462 metres, lies slightly to the north-west and overlooks the cairn, the site itself is conspicuous from a wide arc of sea and land. That an ancient cairn, probably prehistoric in origin, was pressed into service as a convenient platform for an eighteenth-century surveying monument is an unremarkable practical decision, but it has left the older structure in a condition that is part-preserved and part-dismantled, the two phases of use now permanently overlaid on one another.