Measuring roughly twelve metres along its north-west to south-east axis and eight and a half metres across, it is a substantial prehistoric monument, the kind of carefully accumulated stone mound that was almost certainly raised as a burial marker or territorial signal, visible across considerable distances in clear weather. What gives it an additional, slightly incongruous layer of history is a detail recorded on the Ordnance Survey six-inch map of 1898: a trigonometric point was marked here, meaning that Victorian surveyors, working to fix the precise geometry of the Irish landscape, chose this ancient heap of stones as a convenient high point for their calculations.

The cairn does not stand alone on the ridge. A second cairn occupies the south-eastern end of the same summit, suggesting that whoever raised these monuments understood Tully Mountain as a significant place, perhaps marking both ends of a high corridor with structures that declared presence or commemorated the dead. Cairns of this kind are a recurring feature of the Irish upland landscape, and their builders left no written record of intent, so their precise purpose remains a matter of inference. What is clear is that someone, at some point in prehistory, organised considerable effort to haul and stack this volume of stone at altitude, with the blanket bog gradually encroaching around the base in the centuries since.