A rath is a ringfort, a roughly circular enclosure built from earthen banks, used in early medieval Ireland primarily as a defended farmstead. Three-rampart examples are comparatively rare and generally associated with higher-status occupants. What survives today is considerably reduced from what once stood: a subcircular area of roughly 45 metres in diameter, enclosed by a stone-faced earthen bank standing about 1.7 metres in height on the interior, with an internal fosse, a defensive ditch, running from the south-west round to the north-north-west. A stream crosses inside the bank in the south-east quadrant, felled trees and overgrowth crowd the interior, and disused farm buildings occupy the northern side.

By 1890, when the antiquary Barry recorded the site, the attrition was already well advanced. He noted that only one-quarter of the middle rampart and nearly all of the inner fosse remained intact, while a further quarter of the middle rampart and a remnant of the outer rampart had already been cleared away by a Mr. Donovan some twenty years before that. The losses Barry described were therefore largely the work of the mid-to-late nineteenth century, when land improvement and agricultural consolidation swept away a great many such monuments across Munster. What makes Ballyknock particularly layered, even in its reduced state, is the presence of two further features within the enclosure: a souterrain in the north-west quadrant, and the recorded site of a castle in the interior. A souterrain is an underground stone-built passage or chamber, common in early medieval ringforts, typically used for storage or as a place of refuge. The castle site suggests the location retained some strategic or social significance into the medieval period, with the earlier enclosure either reused or built upon by later occupants.