It is easy to walk past without a second glance, but that unremarkable hollow may be all that remains of a souterrain, an underground stone-lined passage built during the early medieval period, typically used for storage, refuge, or concealment.

The depression sits within a rath, the circular earthen enclosure that was the standard form of farmstead in early medieval Ireland, usually consisting of a raised bank and ditch enclosing a domestic area. Souterrains were commonly dug within or adjacent to such enclosures, their roofed passages occasionally extending for considerable distances beneath the ground. At Carrowcoller, the roof has long since given way, leaving only the tell-tale linear sinking of the soil as evidence that something once ran underground here. The dimensions, the orientation, and the location within the rath all point towards a collapsed passage, though the evidence is tentative rather than conclusive.