The opening is blocked, but drawings survive to show what lies behind it: a short passage leading down three steps to a chamber roughly six metres long, one and a half metres wide, and three metres high, its drystone walls inclining inward to meet a flagged roof. A souterrain, for those unfamiliar with the term, is an underground stone-built passage or chamber, constructed during the early medieval period and associated with settlements and ringforts across Ireland, likely used for storage, refuge, or both. This one is unusual not just for its scale but for the precision with which its proportions have been recorded against the silence of its sealed entrance.

Cathair Fionnúrach is one of a cluster of cashels and isolated clocháns, the small dry-stone beehive huts common to this part of the Dingle Peninsula, that sit along the lowest western reaches of Brandon Mountain. The souterrain was accessed from a recess at the base of the north-east wall of the smaller hut within the complex, with a passage of about 2.7 metres leading from that opening before the three descending steps opened into the main chamber running at right angles beneath. The site looks out over the flat plain drained by the Feohanagh river, and on a clear day the view carries west as far as the Blasket Islands. According to work by Curran, cited in J. Cuppage's 1986 archaeological survey of the Corca Dhuibhne peninsula, the walls of the chamber were of drystone construction, carefully built to narrow as they rose toward the flagstone ceiling. The survey remains one of the key records for understanding the density of early medieval activity across this landscape.