A souterrain, from the French for "underground passage", is a man-made underground chamber or series of chambers, typically built during the early medieval period in Ireland for storage, refuge, or ventilation of nearby dwellings. This one gave up its secrets only because something had gone wrong: a capstone, one of the large flat stones forming the roof, had collapsed, leaving an opening that allowed a person to drop down inside.

When archaeologist David Sweetman inspected the souterrain in 1975, he was able to enter through that accidental gap. What he found was a chamber, and from its eastern side, a passage leading further into the ground. That passage, however, could not be followed; it had become choked with infill over the centuries, its contents and extent unknown. A second chamber was also identified, this one built from small stones rather than the larger slabs typical of souterrain construction elsewhere. The contrast in building materials between the two chambers hints at either different phases of construction or different builders working to hand with whatever the local ground could offer.