It is a modest word for what was almost certainly a souterrain, an underground passage or chamber built into the earth, typically during the early medieval period and usually associated with a nearby ringfort. Such structures served various purposes, from food storage to refuge, and this one appears to have belonged to the ringfort immediately above it.

The ringfort at Toanreagh sits at a level noticeably higher than the surrounding land, its interior accessed through a broad southern entrance roughly eight metres wide. Inside, two earthen mounds survive. The larger of the two, towards the north, measures about ten metres by ten internally; the other, positioned to the south-east, is smaller at around four by six metres. These mounds, their shapes softened now by time and vegetation, are thought to be the collapsed or filled remains of the souterrain recorded as a cave by the nineteenth-century surveyors. C. Toal's North Kerry Archaeological Survey, published in 1995, documented both the ringfort and its probable underground feature together, drawing a line between the cartographic label of 1842 and the physical evidence still visible on the ground.