That gentle arc, sweeping from the south-west around to the north, is not the work of a surveyor drawing a convenient straight edge across a map. It is the surviving outline of an early medieval ringfort, a circular enclosure whose bank has been so thoroughly absorbed into the landscape that the monument itself never appeared on the Ordnance Survey six-inch map of 1837, nor on the revised twenty-five-inch edition of 1913. The ringfort is, in effect, invisible on paper, yet still legible on the ground and from the air.

A ringfort, sometimes called a rath, was a roughly circular enclosure defined by an earthen bank and outer ditch, used in early medieval Ireland primarily as a farmstead or high-status residence. At Rathduff, the enclosure sits on a west-facing shoulder of rising ground, with open views to the west and south, a position typical of sites chosen for both practicality and a degree of natural advantage. When the site was examined in 1980, a partially levelled bank of earth and stone, standing to roughly one metre in height, was still traceable from the south-west around to the north, and faint traces of an external fosse, the shallow defensive ditch that once ran outside the bank, remained visible, best preserved at the north-east. The bank to the north-east and east had been more heavily disturbed. What survives most clearly is the curvilinear townland boundary itself, approximately 58 metres in arc, which has inadvertently preserved the western portion of the original enclosure by giving it an administrative function that outlasted its original one.

The full outline of the enclosure, roughly 68 metres north to south and 64 metres east to west, becomes most legible when seen from above. An oblique aerial photograph taken on 17 July 1966, held under the reference CUCAP APH002, shows a cropmark tracing the remainder of the circuit from the north around through the east and south to the south-west, where it meets the surviving bank. Cropmarks appear when buried ditches or disturbed soil affect the growth of surface vegetation, leaving patterns that are invisible at ground level but readable from the air. The site occupies the south-west end of a rectangular field, and its outline remains clearly visible on more recent aerial imagery, a circle quietly persisting beneath the grass of improved pasture.