Stretching for roughly 2,400 metres across the eastern side of the Curragh in County Kildare, the Black Ditch is easy to overlook. It rises only about 1.5 metres, a low earthen bank accompanied by a shallow fosse, a defensive or boundary ditch, running along its western side, with intermittent traces of a second fosse on the eastern side. It is not dramatic in scale, but its quiet persistence across one of Ireland's most historically layered landscapes raises questions that are harder to answer than they might seem.

The bank does not run in a perfectly straight line. Its northern half follows a NNW-SSE orientation for approximately 1,250 metres before shifting slightly to run more directly north-south for the remaining 1,150 metres or so. That subtle change of direction suggests the earthwork was not simply thrown up in a single effort with a single purpose, or that it was made to follow the natural contours and practical demands of the terrain. The Curragh itself, a broad open plain of some 5,000 acres, has served as common grazing land, a military training ground, and a ceremonial landscape since early medieval times, and linear earthworks of this kind are generally associated with territorial marking or the management of livestock and movement across open ground. Whether the Black Ditch fits comfortably into any one of those categories remains genuinely uncertain.