What makes it slightly odd is its form: kidney-shaped rather than the more typical circular or oval profile associated with prehistoric earthworks, measuring about 17.6 metres east to west and 13 metres north to south, and rising only 1.6 metres from the surrounding wetland. More curious still is a distinct opening on its south-eastern side, nearly five metres wide and six metres deep, which gives the mound an almost scooped-out appearance when approached from that direction.

The mound appears on the 1937 Ordnance Survey six-inch map, marked with hachures, the small radiating lines cartographers used to indicate raised ground or earthworks, which tells us it was a recognisable feature in the landscape at least by the mid-twentieth century. Its age and precise function remain unclear. Earthen mounds of this kind in Ireland range from prehistoric burial monuments to medieval ringfort remnants to more recent agricultural or drainage features, and the waterlogged setting at Clashelane does not immediately narrow the field. The recess to the south-east is the detail that most invites speculation, though without excavation it is difficult to say whether it is an original structural feature, the result of later digging, or simply the way the ground has settled over time.