The earthwork at Ballinvuskig is something slightly different: a pentagon. That five-sided outline, measuring roughly 57 metres north to south and nearly 55 metres east to west, gives this grassy mound in a pasture field an angular, deliberate quality that sets it apart from the softer curves of its more familiar cousins.

The enclosure sits on a south-east-facing slope and is defined by an earthen bank that still reaches about 1.5 metres above the outer ground surface on most of its circuit. Around the east, south, and west sides, an external fosse, essentially a defensive ditch dug to heighten the apparent height of the bank above it, survives at depths of up to 0.7 metres, though the southern section has been partially filled over time with stones cleared from the surrounding fields. A counterscarp bank, a low secondary ridge thrown up on the outer lip of the ditch, is also present to the south. The northern side of the enclosure behaves oddly, kinking into two straight sections rather than following any curve at all, and the inner face of that northern bank retains some stone facing, suggesting at some point a more formal revetment was intended or partially built. Entry was provided by a causewayed gap to the east-south-east, a causeway being a section where the ditch was simply left uncut to allow passage through, here just 1.6 metres wide. The interior itself slopes downward toward the south-east, which would have influenced drainage and the placement of any structures once inside.

The site sits in ordinary grazing land and the bank on the eastern side is now grass-covered and topped with a wire fence. The southern bank has been broken in two places. None of this detracts from the overall legibility of the shape, which reads clearly from the ground as something planned, bounded, and very deliberately made.