The island is called Knocknacappul, and the stones occupy its western side, standing at varying heights, the tallest reaching 2.4 metres and the shortest barely clearing 0.35 metres above the ground.

A four poster is a class of prehistoric stone setting found mainly in Scotland and Ireland, consisting of four upright stones placed at the corners of a rectangle or square, sometimes enclosing a burial deposit. The arrangement at Knocknacappul follows the type closely. The long sides of the quadrangle run east to west, and the four stones differ noticeably in height, the north-west stone being the most imposing at 2.4 metres, with the north-east stone reduced to little more than a stump. Adding a further layer of interest, a separate monolith stands 13.3 metres to the north of the group, measuring roughly a metre in length and 0.75 metres above ground. Whether this outlying stone is directly related to the four poster or represents a separate, perhaps earlier or later, act of placing is not clear, but its proximity is unlikely to be coincidental. The site was recorded by the archaeologist Seán Ó Nualláin in 1984, as part of a broader survey of Irish stone settings.

The island setting gives the monument an unusual character. Knocknacappul sits on the southern shore of Kilmakilloge Harbour, and the stones are surrounded by coniferous woodland, which filters the light and muffles the sounds of the coast nearby. The combination of island isolation, a conifer canopy, and a Bronze Age stone arrangement measuring itself out in precise geometry makes this one of the quieter curiosities of the Beara Peninsula.