Roughly 24 metres east to west and 22 metres north to south, it survives as little more than a low, slumped undulation in the ground, rising no more than about half a metre at its most legible points. A faint raised rim along the eastern arc is the clearest surviving trace of what appears to have been a bank, the kind that once defined an enclosed space. It never appeared on the Ordnance Survey six-inch maps of 1838 or 1929, which means it was either already too degraded to be recorded by the nineteenth century, or simply overlooked. Its existence has been pieced together from the slight but telling irregularities in the pasture ground.

What makes the site quietly compelling is its position. The ridge top is high and exposed, and the enclosure sits close to its western end, conforming to the natural roll of the terrain rather than imposing a strict geometry on it. The interior slopes gently downward from east towards the centre, then levels out in the western half, following rather than fighting the contours beneath it. Around it, the landscape opens up considerably: the Nephin Beg mountain range stretches along the northern horizon, with Nephin Mountain prominent, and to the south-west the ridge commands a partial view of Clew Bay, with Croagh Patrick's outline visible beyond. Immediately to the south lies a smaller steep-sided ridge recorded on the 1929 map as Barrett's Hill, and within 160 metres to the east along the same ridge top there is a possible rath, a type of enclosed farmstead typically associated with the early medieval period in Ireland. Further enclosures and a rath lie on the north-facing slope of Barrett's Hill, between 150 and 210 metres to the south-south-west. The clustering of these features across the same narrow ridge system suggests this elevated ground had a sustained significance over time, though precisely when or how the Knocknageeragh enclosure was in use remains unresolved.