It looks, at first glance, like any of the ancient cairns that punctuate the Irish upland landscape, the kind of loose stone mound raised over a burial thousands of years ago. But this one has a more bureaucratic origin, and is no less interesting for that.

The structure appears to date from the nineteenth century, when the Ordnance Survey was engaged in the enormous task of mapping Ireland in systematic detail for the first time. As part of that work, the OS established trigonometrical stations, fixed reference points from which precise measurements could be taken across the landscape. A trig station typically includes a bench mark, a carved or built reference point recording an exact elevation above sea level, and this cairn on Garranbaun served exactly that purpose. The irregular stonework, built up into a rough tower rather than simply piled, reflects the practical need to create something visible and stable on exposed ground. The cairn also functions as a county boundary marker, and it does not stand alone in that role. Smaller piles of stones to the north and south continue the line, like a series of quiet punctuation marks running across the mountain, tracing the administrative boundary that divides the two counties.

The summit position means the cairn is visible from some distance on a clear day, and the views in all directions give a good sense of why surveyors chose such elevated ground for their reference points. Visitors walking the ridge should look for the subsidiary stone piles stretching away to either side, easy to mistake for natural scatter but in fact deliberate markers, each one part of the same nineteenth-century effort to pin the county line to the land itself.