It measures roughly eight and a half metres along one side, incorporates a scatter of boulders, and rises less than a metre above the surrounding ground. Nothing about it announces itself. Yet beneath or beside this modest earthwork, a souterrain once lay hidden, undisturbed and unrecorded on any Ordnance Survey map, until someone came across it in the 1950s.

A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval settlement in Ireland, and used variously for storage, refuge, or both. The one at Ardcost, sitting in low-lying land about a hundred metres south-west of Ardcost Foot, was discovered during that decade and subsequently filled in, which means the underground structure itself is no longer accessible or even visible. What remains is the surface bank, and a local name: the site is known in the area as an óin, an Irish term for a place of that type. The name is itself a small piece of evidence, suggesting that some memory of the site persisted in the community long before its formal discovery, even if the precise nature of what lay underground had become unclear over time.