It measures about 25.6 metres north to south and 23.5 metres east to west, enclosed by an earthen bank still standing some 2.1 metres high, with a fosse, an external defensive ditch, running along its south and south-eastern edges. What makes this particular example quietly interesting is the survival of a second bank, reaching 1.5 metres in height, along the south-western to west-north-western arc, with a berm, a flat shelf of ground, preserved on its inner face. The interior has since been planted with trees, which gives the whole enclosure an oddly wooded character unusual for a field monument of this kind.

This is a rath, the Irish term for a ringfort, the most common class of monument in the Irish countryside. Ringforts were enclosed farmsteads, built and used mainly during the early medieval period, roughly the fifth to twelfth centuries, by farming families who constructed earthen banks and ditches around their homes and livestock. The double-bank arrangement at Kilnahera suggests a site of some local consequence; single-banked examples are far more numerous, and the additional enclosure would have required considerably more labour to construct and maintain. A levelled section visible to the east-south-east as a slight rise in the ground indicates that the outer circuit was not uniformly well-preserved even by the time the site was first recorded, with that portion having been reduced, probably through centuries of agricultural activity on the surrounding land.