At Castlebrown in County Kildare, a D-shaped earthwork sits quietly in level pasture at the foot of an east-facing ridge, its outline now so worn that only a low scarp, at most 0.8 metres high, marks where something more substantial once stood. The straight western side, running about 20 metres from northwest to south-southwest, gives the enclosure its flattened, slightly awkward shape, as though a circle were interrupted mid-construction or gradually eroded along one edge.

The fuller picture emerged not from fieldwork but from the air. A 1965 aerial photograph revealed a cropmark, the faint discolouration of growing crops above buried features, tracing a roughly circular area with an estimated maximum diameter of around 35 metres. That original footprint was considerably larger than what survives on the ground today, where the interior diameter measures approximately 25 metres east to west. The culprit for much of the damage is the small southward-flowing stream that once curved through the western portion of the enclosure. At some point it was recut as a field drain, a practical agricultural intervention that in the process truncated the monument between the south-southwest and northwest. What remains is a poorly preserved fragment of an enclosure whose original form and purpose remain unrecorded, sitting on gentle pasture beneath the long slope of a north-south ridge.