A rath, as these earthwork enclosures are known, was typically a circular bank-and-ditch construction used as a farmstead or settlement during the early medieval period, roughly 500 to 1000 AD. Thousands were built across Ireland, and this one at Grenagh sits on a south-east-facing slope in pasture, still broadly circular, still partially upstanding, though the centuries have not been entirely kind to it.

The 1846 Ordnance Survey six-inch map recorded it as a roughly circular enclosure of around 50 metres in diameter, ringed by trees. By 1939, the same cartographic series showed it slightly reduced, at about 45 metres across. Today, what remains is an earthen bank, still measuring nearly 7.6 metres wide in places and rising to an external height of around 1.6 metres on the north-west to north arc, though the interior face has been worn down to little more than 0.2 metres. The north-east sector has been cut through by a farm trackway and swallowed by agricultural buildings, and the bank itself shows the wear of cattle and encroaching tree roots. The interior ground is uneven, the surface having been disturbed by grassed-over dumped material over the years. A modern brick shed sits just inside the bank at the northern end, and a field boundary runs outward from the southern arc. What lifts the site beyond a simple earthwork, however, is the presence of a souterrain in the south-east quadrant. A souterrain is an underground stone-lined passage or chamber, typically associated with ringforts and thought to have served as storage, refuge, or both. Their construction required considerable effort, which suggests this particular enclosure was once a site of some local significance, even if it now shares its ground with farm buildings and grazing cattle.