It barely reaches a metre in height, measures roughly nine and a half metres north to south and seven and a half metres east to west, and at first glance could pass for a natural feature of the shoreline. It is not. This is a crannog, the term for an artificial or partly artificial island dwelling used in Ireland from prehistory through to the early modern period, and it is an unusually modest example of the type.

What makes the Ballynakill site particularly curious is its scale and situation. Most crannogs were built as substantial platforms in open water, intended as defensible homesteads reached by boat or causeway. This one, by contrast, was originally constructed in just one to two metres of water and was close enough to the western shore to be almost shoreline furniture. Nineteenth-century Ordnance Survey maps still depicted it as a small islet, but the lake level has since dropped enough to leave it stranded on the foreshore, the ancient waterline now some fifty-one metres to the southwest. The cairn itself is built from medium-sized angular and water-rolled limestone blocks, with a possible defining kerb of larger boulders along its northeastern edge, facing out towards the open water. It sits on a slightly raised natural ridge running northwest to southeast, which the builders may have exploited to give the structure added height. Researcher Aidan O'Sullivan, who examined the site in detail, noted that 800 metres to the northeast, directly across the lake at Faughalstown, there is a second crannog of quite different character, a small but high-cairn type. The pairing is suggestive. O'Sullivan proposed that the two sites may not have been residential at all, but may instead have served as markers for a crossing point of the lake, a pair of fixed reference points guiding travellers across open water in a landscape where routes were not always obvious.