Nearly thirty metres across and standing close to six metres high in places, Carnbower is conspicuous from a distance, but it is what lies beneath the surface that makes it genuinely unusual. Three arcs of internal walling are visible on the cairn's upper surface, north and west of its centre, hinting that what visitors see today is not the monument as it was originally conceived, but the result of successive phases of enlargement, each one burying the previous structure within a growing mass of stone.

This pattern of staged growth has a parallel about twelve kilometres to the south-east, at Poulawack, where a Linkardstown-type cairn, a style of Neolithic or Early Bronze Age burial monument typically containing a stone-lined cist, was excavated by Hencken and Movius in 1935 and found to have been built up in similar incremental fashion. The outermost arc of walling at Carnbower may also suggest that the cairn was once considerably taller and had near-vertical sides, much like a related monument on Turlough hill, roughly three kilometres to the north-west. The base of the cairn is obscured by collapse, so whether there was any external revetting, the kind of stone kerb or facing wall that would have given it a sharp, finished edge, remains unclear. The bedrock terraces that radiate outward from the cairn to the north, south-east, and south-west show slight depressions, and these may be the scars left by prehistoric quarrying to supply the enormous volume of stone required. Wandering across the same hillside are ancient field walls, now reduced to low, grass-covered mounds, the closest passing roughly thirty metres to the west, suggesting that the land around the cairn was once organised and farmed in ways now almost entirely erased.