The only reason anyone knows it exists is because the road gave way in 1989, opening a brief and accidental window onto a structure that had lain undisturbed for centuries.

A souterrain is a stone-lined underground chamber or tunnel, typically associated with early medieval settlement in Ireland, and thought to have served variously as storage space, a place of refuge, or both. They are often found in association with raths, the circular earthwork farmsteads that dot the Irish countryside, though the surface features of any such enclosure at Killeenduff, if there ever were any visible ones, had long since disappeared before the road subsided. What the collapse in 1989 revealed was that the structure was substantial enough to cause a section of road to drop into it. Sligo County Council filled and repaired the surface, and the site was returned to its former invisibility. It does not appear on any edition of the Ordnance Survey six-inch map, meaning it escaped cartographic notice entirely through the nineteenth and twentieth centuries.

There is nothing to see at this location today. No earthwork, no marker, no exposed stonework. The road above it shows no sign of what lies beneath, and the surrounding landscape gives no indication that anything of historical significance was ever recorded here. The souterrain endures, in all likelihood intact, simply waiting out the traffic above it.