From the west or south-west it reads as a gently domed rise in the grass, easy to mistake for a quirk of the terrain. Get closer and the geometry becomes harder to dismiss: steeply sloping sides dropping nearly three and a half metres on the western face, a flat-topped platform measuring roughly eight by five metres, and stones pushing up through the sod at irregular intervals along the lower slopes. This is a mound barrow, a type of prehistoric funerary monument in which the dead were interred beneath a constructed earthen or stone mound, and whoever built this one chose their ground with care, using the natural fall of the hillside to amplify the monument's apparent height on three sides.

What makes this particular barrow quietly arresting is not just its position but one detail on its flat summit: a cup-marked stone set into the turf on the western half of the top. Cup marks are shallow, roughly circular depressions carved into rock surfaces, found across prehistoric Europe and Ireland and generally associated with the Bronze Age, though their precise meaning remains genuinely unresolved. The mound itself measures approximately 20.8 metres east to west and 16.5 metres north to south. A possible terrace or berm-like feature runs around the base from south to north-west, though this may be a natural undulation rather than deliberate construction. Slight depressions visible at the south-west and north edges of the summit hint at some past disturbance, the kind of casual digging that affected many such monuments over the centuries before their significance was widely understood.

Bartragh Island sits in Killala Bay, and the mound commands a wide view south over the Moy estuary and east towards Enniscrone. The island is tidal and low-lying for the most part, which makes this knoll at its southern end all the more conspicuous, the kind of elevated point that would have been legible from the water and from the surrounding shores for thousands of years.