A ringfort, or rath, is a circular enclosure, typically of early medieval date, formed by one or more earthen banks and ditches; it would originally have enclosed a farmstead, protecting livestock and household alike. The one at Ballynastick is roughly forty metres across in overall diameter, its interior slightly elevated above the surrounding ground, following the natural slope down from west to east.

The enclosing bank is composed of earth and stone, and where it has not been cut away, it reaches about six metres at its base. The best-preserved stretch runs along the northern arc, where the bank still presents a recognisable profile. Elsewhere, the story is one of gradual attrition. A laneway that already appears on the first edition of the Ordnance Survey six-inch map, dated to 1840, cuts directly through the eastern side, destroying the bank and the outer fosse, the defensive ditch that once ran outside the enclosure wall, from the north-east around to the south. A main road has done similar damage at the north, removing the outer fosse along that side entirely. A six-metre gap on the western side may represent the original entrance, though it could equally be a later widening or insertion. Perhaps most telling is a detail tucked into the eastern face of the interior: the possible remains of wall footings belonging to a lime kiln, a small stone furnace used to burn limestone into agricultural lime, built directly into the ancient structure and accessed from the same farm lane that helped destroy it. The kiln speaks to the ringfort's afterlife as a convenient source of ready-cut stone, a fate shared by many such monuments across Ireland.

The site sits in open grassland with long views to the east, and two further related monuments lie within close range, including a cliff-edge fort roughly four hundred metres to the south-west and another ringfort three hundred metres to the south, suggesting this upland area once carried a denser pattern of early settlement than its present emptiness implies.