The structure is a souterrain, an underground chamber or tunnel constructed from drystone walling, without mortar, the stones carefully laid so that the walls and roof hold by weight and friction alone. This one runs roughly northeast to southwest, stretching seven metres in length but only 1.4 metres wide and less than a metre high, dimensions that would require any adult to crouch or crawl. Entry today is through a gap left by a displaced roof lintel, one of the large flat stones that would originally have sealed the ceiling in a continuous line.

The souterrain sits within the southern sector of a rath, the kind of circular enclosure, typically defined by an earthen bank and ditch, that was the standard farmstead form across early medieval Ireland. Raths were domestic spaces, and souterrains were a common feature of them, though their precise function is still debated. Cold storage, refuge in times of raid, or simply a secure place to keep valuables are the explanations most often proposed. What survives at Loughcurra is partially blocked, and access into the chamber is awkward, but the structure retains at least one detail worth noting: an air vent remains visible inside, suggesting that whoever built it gave some thought to what, or who, might need to breathe down there.