The enclosure at Knocknaganny is a cashel, a type of ringfort defined by a drystone wall rather than an earthen bank, and what survives here is reduced to a broad, grass-covered bank roughly three metres wide, tracing a circuit about thirty metres in diameter. A more recent field wall, crudely built from single limestone boulders and no more than eighty centimetres high, has been laid directly over the old cashel wall, running along its inner edge on the eastern side and switching to the outer edge on the west. The effect is a palimpsest of enclosure, one era of boundary-making pressed awkwardly against another.

The ridge itself does some of the structural work. On the northern arc, the cashel wall coincides with the natural break of slope where the ground drops into a small, narrow valley below, meaning that whoever built here used the landscape's own geometry as part of their defences or simply their definition of space. To the south, the Ordnance Survey six-inch map marks an area as liable to flooding, consistent with a turlough, the seasonally flooding lakes characteristic of karst limestone country, where water rises through the porous rock rather than draining into conventional channels. The surrounding landscape is that same karst terrain, grassland interrupted by rock outcrops and open pavement. An aerial photograph captured the cashel alongside a wider field system extending east and south, accompanied by a cluster of rectangular vernacular houses. That entire landscape, fields and buildings together, was subsequently cleared during land reclamation works, leaving the cashel as the sole visible remnant of what had been a more complete agricultural settlement.

Inside the enclosure, a plantation of conifers now makes close examination difficult. The ground beneath them is uneven, covered in pine needles, with scattered boulders and small stone heaps suggesting subsurface features that have never been properly examined. In the north-west quadrant there is a rectilinear depression, approximately six metres by three, and around forty centimetres deep, which may represent the footprint of a house. The trees that obscure it are the most recent layer in a long sequence of occupation, abandonment, and transformation on this quiet Mayo ridge.