The earthwork here occupies a southeast-facing slope in pasture, and while much of its encircling bank was reportedly flattened around 1890 on what was then Pat Murphy's land, a circular area roughly 30 metres across remains legible on the surface. A low scarp still defines the southern, western, and northern edges, and to the east an earthen bank, standing about 1.5 metres on its outer face, has been absorbed into the field boundary system rather than removed. A shallow fosse, barely 0.2 metres deep, runs along its exterior. The form is partially legible, partially lost, a common condition for ringforts across Ireland, where centuries of agriculture have worn away earthworks that the Ordnance Survey cartographers were still carefully hachuring as late as 1938.

Ringforts, sometimes called raths, were enclosed farmsteads typically built during the early medieval period, roughly the fifth to twelfth centuries. They were the most common settlement type in early Ireland, with tens of thousands believed to have existed. The Knocknashannagh example appears on the 1842 Ordnance Survey six-inch map as a hachured circular enclosure, and successive editions of the map chart its slow degradation. Bowman, writing in 1934, recorded it as a levelled fort with a diameter of approximately 28 yards, while Broker, in 1937, noted the same levelling event and placed it around 1890. What makes the site quietly unusual is the cup-marked stone at the southern side of the interior. Cup marks, shallow rounded depressions pecked into rock, are generally associated with prehistoric activity, considerably older than the ringfort period, which raises the possibility that the site had a longer history of use, or that the stone was brought here from elsewhere. A second ringfort sits in an adjoining field only about 20 metres to the east, a pairing that is not unheard of but far from routine, and which suggests this particular slope was considered a worthwhile place to settle by more than one community.