The word is a practical understatement. What lies beneath the turf at Derryronan is a souterrain, an underground stone-built passage of the kind constructed throughout early medieval Ireland, typically for storage, refuge, or both. At ground level the entrance is modest: an opening roughly two metres east to west and one metre north to south, with a single roof lintel visible at its southern edge. Below that threshold, a passage extends southward for about 4.2 metres. It is narrow, barely 0.8 metres wide, and low, standing around 1.1 metres high, so any exploration would require crouching. The walls are built from rough, rounded stone rubble, and the roof is formed by large flat slabs laid in a horizontal row, contiguous or slightly overlapping, in the dry-stone technique characteristic of these structures.

The souterrain sits within the northern half of a ringfort, the type of enclosed farmstead that formed the basic unit of rural settlement in early medieval Ireland, typically defined by one or more circular earthen banks. Souterrains were commonly incorporated into such enclosures, and the combination at Derryronan fits a pattern found widely across the country, though each example carries its own peculiarities. Here, the earthen floor of the passage rises sharply as it approaches the southern end, eventually meeting the level of the lintels above it, where a narrow gap opens back up to the surface. Whether that rising floor reflects the original design or is partly the consequence of soil collapse and gradual infill over the centuries is unclear. Local tradition, however, holds that beyond this constricted southern end there is a small chamber, a detail that has not been formally verified but is consistent with how souterrains elsewhere were built, often incorporating side chambers or terminal rooms off the main passage.