A souterrain is an underground stone-lined passage or chamber, typically built during the early medieval period and associated with ringforts as a place of refuge, storage, or concealment. This one has no visible surface trace whatsoever, meaning there is nothing above ground to indicate its existence at all.

The only evidence comes from a brief observation made in 1937, when a writer named Broker noted an opening in the middle of the ringfort that had, even at that point, already fallen in. That collapse has since been total. The souterrain sits within a ringfort recorded separately, and the two together represent the kind of layered early medieval settlement that appears across Cork and the wider Irish countryside, where a circular earthen enclosure would have enclosed a farmstead and its associated structures. The underground passage belonging to such a site might have stored dairy produce, provided a bolt-hole during raids, or served some combination of practical purposes that scholars continue to debate. At Dooneens, none of that function is now accessible, even to interpretation on the ground.