What looks at first like a simple raised ring is, on closer inspection, a layered piece of early medieval engineering: a roughly circular enclosure, close to 28 metres across, wrapped not by one earthen boundary but by several distinct features working together.

Ringforts, known in Irish as raths when built primarily from earth rather than stone, were the standard farmstead form across Ireland from roughly the fifth to the twelfth centuries. Most enclosed a family's house and outbuildings within a raised bank, offering security against livestock theft and marking social status as much as providing defence. The Grenagh example is more elaborately constructed than many. A scarp, meaning a steep face cut into or built up from the natural ground, rises to a maximum of one and a half metres and defines the inner enclosure. Along part of the western to north-eastern arc, a slight internal lip adds a further raised edge, reaching about 0.6 metres. Beyond the scarp, a fosse, which is a rock-cut or earthen ditch, encircles the whole, and outside that again stands a substantial outer earthen bank reaching 1.6 metres in height. That outer bank survives all the way round, which is notable given how thoroughly farm improvement has erased such features elsewhere in the Irish countryside. To the east, the scarp becomes less pronounced, and the outer bank carries a gap two metres wide, crossed by a causeway, almost certainly the original entrance point.