At Clooncous in County Mayo, quarrying along the southern and western flanks of an ancient burial mound has accidentally uncovered a glimpse of the internal structure that prehistoric builders went to considerable effort to conceal. The result is an oddly instructive ruin, part intact monument, part accidental cross-section.

The tomb is a wedge tomb, a type of megalithic monument built during the late Neolithic and early Bronze Age, typically consisting of a tapering stone-walled gallery covered by capstones and buried beneath a cairn of loose rubble. At Clooncous, that cairn is sub-circular in plan, measuring roughly 17.2 metres east to west and 14 metres north to south, and still rises to nearly 1.8 metres at its eastern end. Beneath it, aligned northeast to southwest, runs a narrow gallery just 0.8 metres wide. Where quarrying has cut into the western side, about three metres of that gallery is now exposed. Three large upright boulders, known as orthostats, form part of the western wall, with the tops of their counterparts on the eastern side just visible above the accumulated earth inside. Three roof slabs still span the gap between the two rows of uprights, and the gallery fill sits only 0.25 metres below those capstones, suggesting much of the original structure survives intact beneath the mound. Around the cairn's perimeter, stones project outward in what looks like an irregular scatter, though along the north-northeast edge there is a more deliberate arrangement that may represent the remains of a kerb, the low retaining wall commonly used to define and stabilise such cairns. The monument sits on a low rise in undulating pasture, with Nephin Mountain visible to the west and the Ox Mountains extending across the northwest and north, and a stream running roughly fifty metres to the west.