The ring, roughly 26 metres across, is not defined by any visible earthwork, no raised bank, no ditch, no wall. The thistles simply grow in a closed loop, as if responding to something beneath the soil that human eyes and local memory have both lost track of.

The site came to attention in September 1991, when aerial photography carried out as part of a systematic survey of the Irish landscape captured a circular shadow mark at this location. Shadow marks of this kind appear when buried or disturbed soil retains moisture differently from the surrounding ground, causing subtle variations in vegetation colour or growth that are invisible at ground level but legible from the air. In this case, what the camera recorded has a counterpart on the surface only in that band of thistles, plants that are known to colonise disturbed or nutrient-altered ground and can persist for generations along the edges of long-vanished structures. No earthwork survives, and unusually, there is no local tradition attached to the spot, no folk name, no story, no memory of what the circle might once have been. An enclosure in Irish archaeology typically refers to a defined area set apart by a bank or ditch, associated with settlement, ritual, or agriculture, but at Oldcastle none of those categories can be applied with any confidence. The site remains unclassified in any meaningful sense, known only by its shape and by the quiet insistence of the plants that outline it.