That double-ring plan, familiar enough in early medieval Ireland, has since been levelled almost entirely. What remains are three low mounds of earth and stone to the south-east, a steep natural scarp that once doubled as the western and northern defences, and, in the south-west quadrant of the vanished fort, two collapsed openings leading into something considerably stranger: a stone-built souterrain passage, roofed with large flat slabs, at least 4.3 metres long and aligned roughly north to south.

A souterrain is an underground passage or chamber, usually dry-built in stone, associated with ringforts of the early medieval period, roughly the fifth to twelfth centuries. They served variously as places of refuge, cool storage, or escape routes. The one at Rossacoosane is now largely infilled, its southern opening measuring just under a metre wide and 0.8 metres high, with barely 0.4 metres of clearance remaining between the rubble fill and the capstones above. The overall ringfort diameter was around 30 metres, modest by Kerry standards but entirely typical of the single-family farming settlements that once punctuated this landscape in their thousands. The place may be the same site listed in the Ordnance Survey Name Books under the Irish names Cnocán an Dúinín or Cnocán na gCoinín, where a local informant described it plainly as a fort with a cave in it. That description, probably recorded in the 1840s, turns out to be more precise than it sounds: what survives underground is exactly that, a cave of carefully laid stone, waiting beneath a field that shows almost no trace of the enclosure that once surrounded it.