Somewhere on the open grassland of the Curragh, a low earthen ring sits on a gently eastward-sloping plain, unremarkable at ground level but distinctly legible from the air. What makes it genuinely odd is the concrete pill-box, a small defensive fortification of the kind associated with twentieth-century conflicts, built directly into the enclosing bank on its south-western side. The combination of an earthwork that reads like something ancient and a hardened military structure from a completely different era produces a quiet dissonance, the sort of thing that raises more questions than it answers.

The enclosure is roughly sub-circular, measuring around eleven metres north to south and nine metres east to west, ringed by a broad earthen bank and a narrow outer fosse, which is simply a ditch dug to define and defend the perimeter. A gap in the bank at the west-north-west, with a corresponding causeway across the fosse, forms the main entrance to the interior. A second, narrower causeway to the north appears to be a later addition, probably formed when spoil from a pit dug just outside the site was deposited in the fosse rather than cleared away properly. The site does not appear on any edition of the Ordnance Survey six-inch maps, which is itself significant: features that predate the nineteenth-century surveys tend to appear on those maps even when poorly understood, so an absence suggests the earthwork may be comparatively recent. The prevailing assessment is that it is some sort of military earthwork, consistent with the Curragh's long history as a military training ground. It was identified from a Department of Defence aerial photograph taken in 1999, which is how such low, grass-covered features are most reliably spotted; from the ground, the banks rise only about a metre above the interior surface.