What was once a substantial fortification, recorded on Ordnance Survey maps as early as 1842 under the curious name 'The Bone Fortification (in ruins)', has sunk gradually into the earth over the centuries until its walls, once metres thick, now survive only as low, wide banks and exposed boulders. The 1916 OS map adds another layer of intrigue by marking a cave in the south-eastern corner of the complex, a feature that has left no obvious explanation behind it.

When the scholar John O'Donovan visited in 1841 as part of the Ordnance Survey Letters project, he found the structure still impressive enough to be worth describing at length. He noted walls eight feet high and seven feet thick, and so many portholes that he gave up trying to count them all. Charles Smith, writing in 1756, described it simply as 'a square stone building', which suggests either that the full complexity of the layout was already obscured, or that he saw only part of it. What survives today is considerably more elaborate than either description implies: two rectangular enclosures in the western part of the field, measuring roughly 21 by 36 metres and 24 by 33 metres internally, sharing a common central bank, alongside a roughly circular enclosure in the south-east measuring about 34.5 metres across, with stone banks radiating from its corners and connecting to the broader system of earthworks. A low bank curves away from the main field boundary, runs for some 22 metres, then bends back before descending towards a small watercourse known as the bone rivulet. The whole complex spans the field it occupies. It has been suggested that the fortification may have been built by the Cantillon or D'Cantillon family of Ballyheigue, a local Hiberno-Norman dynasty. There is also speculation that 'bone' is a corruption of 'bawn', a term for a walled enclosure, typically used to protect cattle, which would reframe this sprawling structure as something more agricultural than military in its original purpose, though the two were not always easily separated in early Irish land management.