A souterrain is a stone-lined subterranean passage, typically associated with early medieval ringforts and used for storage or refuge. No surface trace of it survives today, which is part of what makes this quiet earthwork quietly unsettling: the ground keeps its own counsel.

The site at Lerrig is a univallate rath, meaning it has a single enclosing circuit rather than the multiple concentric rings found at more elaborate examples. A roughly circular area some thirty metres across is ringed by an earthen bank approximately six metres wide, which still rises to around 1.2 metres above the interior and nearly two metres above the outer ditch, or fosse, that once ran alongside it. That fosse is now largely levelled; along the northern, north-eastern, and southern arcs it has vanished entirely, worn down by centuries of agriculture or weathering. Where it does survive, it is between two and three metres wide and about 1.2 metres deep. The interior sits noticeably higher than the surrounding land, a characteristic feature of these earthen enclosures that gives even a heavily eroded example a presence in the landscape. From this slight elevation, the views extend in every direction, which was almost certainly the point when the site was first constructed, most likely sometime in the early medieval period when raths served as the enclosed farmsteads of local farming families.