The Knockbrack ringfort still rises above the surrounding land as a circular platform roughly thirty metres across, but the feature that makes it particularly curious is what it conceals rather than what it displays: a semi-circular depression in the northwest sector of the interior, pressing against a stone-faced bank, marks the blocked entrance to a souterrain, an underground stone-lined passage of the kind commonly associated with early medieval settlement in Ireland, used variously for storage, refuge, or both. The landowner, a Mr Collis, confirmed the identification, though the entrance was filled in some years before the site was formally surveyed.

The enclosure itself is a univallate rath, meaning it was defended by a single enclosing bank and external fosse, the fosse being a surrounding ditch dug to reinforce the barrier. At its best-preserved point, to the east, the fosse is 3.8 metres wide and 0.3 metres deep, now very shallow but still legible in the landscape. The bank has been modified and damaged in places: the northwest to northeast stretch was at some point given a stone facing, while the southwest to west sectors have been disturbed. A causeway of sorts crosses the fosse to the south, though no corresponding gap survives in the bank to suggest a formal entrance ever aligned with it. Five small mounds punctuate the raised interior, three arranged in a line running northwest to southeast, one in the northeast sector, and a fifth towards the east. Their origin and purpose are not recorded. The site was documented in the North Kerry Archaeological Survey, compiled by C. Toal and published in 1995, which captured the rath in its already degraded condition.