The enclosure at Knockduff presents itself as a D-shape, roughly fifteen and a half metres across on its north-east to south-west axis, but the straight side of that D is not original stonework at all. It is a later field boundary, a workaday wall probably built within the last century or two, which happens to cut across the arc of something much older. What survives of the ancient structure is a low, curving wall, barely thirty centimetres high and around a metre thick, set deliberately along the lip of a natural scarp. That scarp, itself a metre and a half in height, is faced with a partially collapsed drystone wall, suggesting the builders were reinforcing a natural feature of the ground rather than simply piling stones across flat pasture.

The suspicion is that this was once a circular enclosure, the kind of roughly circular or oval space defined by a drystone or earthen boundary that appears throughout the Irish landscape and which dates, in most cases, somewhere between the early medieval period and later prehistory. Such enclosures served a range of purposes, from the enclosure of livestock to the definition of a domestic settlement or farmstead. At Knockduff, the geometry of what remains, and the way the curving wall aligns with the scarp, points to a structure that once continued beyond where the later field wall now runs, extending further to the north-east into ground that has since been reorganised by more recent agricultural activity. How much of the original circuit survives beneath or behind that modern boundary is not known.