Somewhere on the flat pasture that borders the Curragh in County Kildare, an ancient enclosure has all but ceased to exist above ground. What makes it quietly remarkable is that its outline survived long enough to be recorded on the first edition of the Ordnance Survey six-inch map in 1838, drawn as a D-shape, roughly 32 metres across from north to south, with an unusually straight southern side stretching some 47 metres. Today there are no visible surface traces; the enclosure lives only in that early cartographic snapshot.

The D-shaped plan was likely not the monument's original form. Three small roofed buildings appear on the same 1838 mapping immediately to the south, and it is thought these structures may have caused the enclosure to be cut back, its southern arc replaced by the straight edge that gave it that flattened profile. Stripped of that distortion, the monument was probably once a roughly circular enclosure, a form common throughout Ireland and typically associated with early medieval settlement, though the notes stop short of assigning it a specific period or function. What the mapping also preserves is an administrative curiosity: the eastern arc of the enclosure was used as a boundary marker, dividing the townland of Tully East in the parish of Tully from the Curragh in the parish of Kildare. It is not unusual for old earthworks to be pressed into service as boundary features; their solid, recognisable lines made them convenient landmarks for those drawing up parish and townland divisions, often centuries after the original structure had lost its purpose.