This is a rath, the Irish term for an earthen ringfort, a class of enclosed settlement built across Ireland predominantly during the early medieval period, roughly between the fifth and twelfth centuries. Thousands survive in various states of preservation, but this one at Castlemagarret retains a surprising degree of coherence in its basic form.

The platform measures roughly 39 metres north to south and just under 35 metres east to west, making it a substantial example of its type. The enclosing scarp, the steeply cut outer face of the raised platform, rises to around two metres at its highest point on the western side, dropping to just over a metre at the south. A fosse, the encircling ditch dug to provide both drainage and a degree of defence, survives as a shallow depression at the base of the scarp, varying between two and three metres wide on the north and west sides. The builders appear to have compensated for a natural slope in the ground by piling the earth higher on the south-eastern side, leaving the interior platform broadly level. The southern face of the scarp is noticeably lower and more gently sloped than elsewhere, which may mark the position of the original entrance, a common feature at the south or south-east of such enclosures. Inside, several narrow elongated depressions in the ground, obscured by long grass when the site was last examined, are thought likely to be badger setts, suggesting the earthwork has long since passed from human use to something more informal.