The souterrain, an underground tunnel or chamber system built in early medieval Ireland, has long since collapsed, but the ground above it has not forgotten. What remains visible is a grassy depression, roughly 26 metres in total length and about 1.8 metres wide, sinking to around a metre in depth at its most pronounced. That shallow trench is the outline of something that was once carefully engineered.

The souterrain sits within a ringfort, the circular earthwork enclosure that was the standard unit of rural settlement in early medieval Ireland, typically comprising a bank and ditch surrounding a homestead. This particular ringfort at Dundoogan is a rath, meaning an earthen-banked example rather than one built from stone. The souterrain begins just inside the rath bank at the north-north-east, runs south for about 9 metres, then turns sharply west for 10 metres, then turns sharply south again for a final 7 metres. The result is a Z-plan, a configuration that was not accidental. The deliberate changes in direction in souterrains of this type are generally thought to have served a defensive or concealment purpose, making any intruder crawl blind into each new section rather than seeing straight through to the far end. Whether these passages were used primarily for refuge, for food storage in the cool underground air, or for both, remains a matter of ongoing discussion among archaeologists.