The largest monument on this already monument-dense hill is a near-circular enclosure nearly 200 metres across, and for most of its circuit it exists only as a magnetic signature in the earth, detectable by geophysical instruments rather than by walking its edge. A segment of the silted-up ditch does survive as a broad, shallow depression near Lough Lugh on the south-west, and the main entrance is still legible on the north-east as a gap of around four metres, its northern side defined by a distinctive inward-curving terminal. But much of the rest has been swallowed by centuries of ploughing and, on the north-west and south, by the dry-stone boundary walls that now divide the townlands of Mweelra, Ushnagh Hill, and Rathnew from one another, the monument straddling all three.

The enclosure was first picked up during geophysical survey in 2005, when its south-western arc came into view. Further work in 2009 traced its full extent, revealing a sub-circular ditch averaging two metres in width, possibly rock-cut in places, though no accompanying bank has been identified. A prominent mound crowns the gentle rise at the enclosure's centre, and geophysical survey has also recorded ring-ditches, numerous pits, and a scatter of less easily interpreted features within and around the circuit. Some half-dozen pit-type features along the line of the ditch itself may represent post-pits or deliberate deposits. The broader pattern, a large hilltop enclosure containing a central mound and associated with an assembly landscape, invites comparison with ceremonial enclosures at Tara, Navan Fort, and Rathcroghan, all of them royal centres of the late prehistoric period. That analogy points, cautiously, toward a possible Iron Age date, though nothing has been confirmed by excavation.