The Ordnance Survey mapped a circular enclosure here in 1838, recording its outline with the confidence of surveyors who could still see something worth drawing. Today, in the rough pasture that covers the slope, there are no visible surface traces left at all. The site is, in the most literal sense, invisible.

The enclosure is thought to be possibly a ringfort, the most common type of early medieval farmstead in Ireland, typically consisting of a raised circular area enclosed by one or more earthen banks and ditches. Thousands survive across the country in varying states of preservation, but many more have been lost to agriculture, land improvement, and the slow settling of the ground over centuries. What the 1838 OS map captured at Knocknakillew may have already been fading then, the earthworks low enough to record but not robust enough to endure another two hundred years of grazing and weather. The gap between that survey and the present is where the site effectively disappeared.