The overall external diameter of the enclosure at Ballynaclin runs to around 115 metres, making it an enormous example of the monument type known as a henge, a roughly circular prehistoric ceremonial enclosure typically defined by a ditch with an external bank, the bank being placed outside the ditch rather than inside as in a defensive earthwork. The sheer scale of what survives is partly down to the trees and shrubs that fringe most of the perimeter; their root systems have locked the earthen fabric in place, keeping erosion at bay in a way that the more exposed northern arc has not benefited from.

When surveyed in detail by David McGuinness in 2012, the site revealed a layered complexity that goes well beyond a simple ring. The central platform, its upper surface measuring roughly 89 metres north to south and 82 metres east to west, appears in places to sit above the surrounding ground level, suggesting that the hilltop itself was deliberately scarped, that is, cut and shaped, in antiquity. On the well-preserved north-western side, the platform rises 2.6 metres above the base of the ditch, which is flat-bottomed, up to 5 metres wide, and separated from the inner edge of the outer bank by a gap of over 14 metres. That outer bank, best preserved on the south-western side, is up to 13.5 metres wide at its base with a broad, flat top. Within the platform, the monument multiplies further. A sub-circular mound, roughly 13.5 by 16 metres in diameter and 0.9 metres high, sits towards the north-western portion of the platform. To the south-east, a roughly D-shaped embanked enclosure, measuring 33 by 19 metres, is set against the inner edge of the main ditch and entered through a neat 2-metre gap on its northern side. The ground inside this smaller enclosure is irregular, with several raised mounds visible to the south of its centre. McGuinness noted at the time of survey that a dense growth of nettles may have concealed further features still. A kettle-hole lake, a small water-filled depression formed by glacial ice, lies in the field to the north-east, in the hollow at the base of the same eminence, giving the landscape around the monument a quietly prehistoric atmosphere even before the earthworks themselves come into view.