A souterrain is an underground stone-lined passage, typically associated with early medieval ringforts and used for storage, refuge, or both. At Rathtrim, what survives above ground is not a passage but its absence: a roughly Z-shaped depression in the turf, first recorded in 1980, that suggests the roof of such a structure has long since collapsed inward. No stonework is visible. The ground simply dips and angles in a way that pasture land ordinarily does not.

The depression sits within the north-western half of the interior of a ringfort, a type of enclosed farmstead common in early medieval Ireland, typically defined by a circular earthen bank and ditch. The ringfort at Rathtrim occupies a low rise in undulating pasture, with open views to the north and north-east, and higher ridges closing in from the south and south-east. That positioning is characteristic: ringforts were generally sited to command reasonable visibility across the surrounding landscape. The souterrain, if that is indeed what the depression marks, would have opened from inside the protected space of the bank, running toward the centre of the enclosure and then angling north toward the interior scarp. The Z-shape implies at least two changes of direction, which is not unusual for souterrains, where bends were sometimes deliberate features, perhaps to slow an intruder or to manage airflow for whatever was stored within.