A section of it caved in at some point, and rather than leave an open hazard, the entrance was blocked up. What remains, sealed away, is a souterrain: an underground chamber built without mortar, stone laid carefully against stone, running roughly east to west for about five metres, and barely a metre wide throughout its length.

Souterrains are a recurring feature of early medieval Ireland, typically constructed beneath or alongside ringforts and cashels as places of refuge, cool storage, or concealment. A cashel is a ringfort built from stone rather than earth, and this one at Caherfurvaus is a known site in its own right. The souterrain sits within its northwest quadrant, a position that would have placed it close to the interior of the enclosure. What little is known of its interior comes from a torch-light inspection carried out at some point before the access was lost, which confirmed the drystone construction and gave the approximate dimensions now on record. References by Redington in 1916 and McCaffrey in 1952 place it within a modest antiquarian literature for the area, though neither account appears to have prompted excavation or conservation before the collapse rendered the question moot.

The chamber is now inaccessible, and there is no indication that this is likely to change. The cashel itself remains the visible feature at this location, its stone enclosure still legible in the landscape even if what lies beneath it has effectively been closed off by circumstance rather than intention.