The structure is a souterrain, a type of stone-lined underground chamber or passage built in early medieval Ireland, typically for storage, refuge, or both. What makes this one quietly interesting is its layout: three chambers branching off in different directions, giving the whole thing a forking geometry that is more complex than the simple linear tunnels that survive elsewhere.

The souterrain is drystone-built, meaning its walls were assembled without mortar, relying on carefully fitted stone to hold their shape. It sits within the enclosure of a cashel, a circular stone-walled fort of the kind that once served as a farmstead or minor lordly residence across much of western Ireland. The first chamber runs east to west and measures roughly three metres long, just over a metre wide, and about one and a half metres high, low enough that any visitor would have needed to crouch. From the eastern end of that chamber's southern wall, a second chamber extends five metres northward and southward, forming the upright of the F. A third chamber, shorter at under two metres and now filled with rubble and collapsed material, branches off southward from near the western end of the first chamber, completing the shape. The total known length of the structure runs to more than nine and a half metres, though the state of the third chamber means that figure is a minimum rather than a final measurement.

The site sits in a part of County Galway where early medieval enclosures are not uncommon, but the combination of cashel and multi-chambered souterrain together makes this particular corner of Termon worth pausing over. The third chamber survives only as a rubble-filled hollow, so the full original extent of the system is difficult to determine with any certainty.