It is trapezoidal in plan, 7.5 metres long, broadening to a façade of 5.3 metres at its southern end and pinching to just 1.5 metres at the north. It rises no more than half a metre above the bog surface at its tallest point, and for much of its length the stonework has to be felt rather than seen, pressing up through the vegetation underfoot. Nobody is entirely certain what it is.

The structure is built from drystone walling and upright boulders, with what appears to be a double-wall construction along the western side, an outer run of contiguous upright stones facing an inner wall of smaller courses. At the narrow northern end, a single upright and two flanking stones form a small box-like chamber, roughly 1.5 metres east to west and less than a metre north to south. In the interior, beneath the heather, there is a suggestion of a narrow central passage or gap running north to south, with a prostrate slab lying across its southern end. These features, taken together, have prompted comparisons with megalithic tomb traditions, particularly the wedge tomb form common in the west of Ireland, where a long stone-built gallery narrows from a broad, often south-westerly facing entrance towards a closed back end. But nothing here is confirmed. Attached to the south-western corner is a small D-shaped hut of rough drystone, approximately three metres across, with a narrow entrance on its eastern side. This hut is thought to be later in date, and may have been constructed using stones taken from the larger structure, which would partly explain why the original walling on the south and west sides is now so difficult to read.

The bog and heather of Slievemore have absorbed the structure so thoroughly that its purpose, date, and even its basic character remain, as the archaeological record puts it plainly, enigmatic. The presence of the later hut complicates matters further, obscuring the relationship between the two features and raising the possibility that the trapezoidal cairn was already a ruin when someone decided to shelter beside it.