That enclosure is a cashel, a type of early medieval stone ringfort, and its presence here on the margin between Berneens and Poulnabrucky townlands suggests it was already a landmark of some consequence long before anyone drew a boundary on a map.

The structure is circular, around 16 metres in diameter, and survives unevenly. Along its south-west to east-south-east arc, a wall of rough flat slabs still stands, with external facing reaching between 0.8 and 1.4 metres in height and a rubble core about 1.2 metres wide. A short stretch of inner facing survives at the north-north-west, though barely. Where the wall has disappeared entirely, between the south-east and south-west, a scarp of 1.6 metres marks its former line, and it is along this earthen edge that the townland boundary curves. The antiquarian Thomas Johnson Westropp, writing between 1900 and 1902, described Caherberneen as a nearly levelled fort, which suggests that even by the turn of the twentieth century the stonework had suffered considerably. It appears by name on the Ordnance Survey six-inch maps of both 1842 and 1916, so its identity, if not its fabric, was never entirely forgotten. The site sits within a multiperiod field system, meaning the landscape around it preserves traces of agricultural organisation from several different eras layered one on top of another.

The cashel occupies a slightly raised area of rough pasture, ringed by hazel and rocky scrub, and commands clear views in all directions. That positioning was almost certainly deliberate; such elevated, open ground would have served both practical and perhaps territorial purposes for whoever built and used it.