A trigonometric point and an Ordnance Survey cairn placed there in the nineteenth century have flattened its crown, leaving the original mound with an unnaturally level top that obscures whatever form it once held. The result is a layered object: ancient stone beneath Victorian cartography, each era having made use of the same high ground for its own reasons.

The cairn is a modest construction, roughly six metres in diameter and between one and one and a half metres in height, built from angular stone in a circular plan. Cairns of this kind are typically prehistoric funerary or commemorative monuments, raised on elevated ground where they would have been visible across the surrounding landscape. At 1,172 feet above sea level, the summit of Tully Mountain would have offered precisely that kind of prominence. When the Ordnance Survey teams moved through Connacht in the nineteenth century, they recognised the same quality in the spot and used it as a triangulation station, a fixed point from which to measure and map the terrain below. Their small marker cairn, piled on top of the original, inadvertently damaged the very thing it was built upon. Researchers Gibbons and Higgins noted the site in 1988, drawing attention to the probability that the flat top is not original but the result of that later truncation.