At Rathumney, the bend in the tarmac traces the eastern boundary of a large enclosure that has otherwise all but vanished from the surface of the land, surviving only as a cropmark, a faint discolouration in growing crops that becomes legible from the air when dry summers stress the soil above buried features.

The cropmark, recorded in an aerial photograph, reveals a curvilinear enclosure running roughly east to west across the hilltop at a span of around 100 metres, with an entrance oriented to the south-east. A fosse, meaning a ditch dug as part of a boundary or defensive perimeter, defines its arc. Within the interior, a second curved fosse runs north to south, though this inner feature may not belong to the same period of use; it appears to align with a ploughed-out field boundary, suggesting it could be a much later agricultural addition. In the adjoining field to the north, a separate but related feature appears: an incomplete circular rath, the term for a roughly circular earthwork enclosure common in early medieval Ireland, defined by its own fosse and a narrow concentric outer fosse running parallel to it. The two enclosures sit close enough, and relate to each other strongly enough in their layout, that they are thought to form parts of a single monument rather than two unconnected sites.

The hilltop setting is typical of enclosures of this kind, which were often positioned to command views across surrounding lowland. What is less common is the way the present-day road appears to have been laid out in deference to the older boundary, bending where the ancient edge of the enclosure once stood, as if the memory of the monument outlasted the monument itself.