These are souterrains, a type of dry-built underground structure common across early medieval Ireland, typically associated with raths and thought to have served as places of refuge, storage, or concealment. What makes Aghavadden quietly notable is the pairing: two of these structures within a single rath, each built to a similar design, occupying different positions within the enclosure.

The more fully recorded of the two sits in the north-western quadrant of the rath. A linear passage, approximately 5.5 metres long and 1.5 metres wide, runs on a north-east to south-west alignment before narrowing sharply into a creep, a deliberately constricted section just 75 centimetres wide and 80 centimetres high at most, designed so that anyone moving through it would be forced to slow down and lower themselves. Beyond the creep lies a small subrectangular chamber, roughly 5.5 metres by 2 metres, with walls of drystone masonry that lean inward toward the top to carry a roof of large flat slabs. The second souterrain, of comparable construction, is built into the thickness of the rath's inner bank at the south-west. A rath, for context, is a circular earthwork enclosure typical of early medieval Irish settlement, its banks and ditches defining a farmstead or homestead. The survey on which these descriptions are based was carried out in 1944.

The passage is largely collapsed now, and the creep that once filtered entry into the chamber is reduced to a narrow, low gap rather than a functioning threshold. The flat slab roofing, where it survives, gives some sense of the original engineering, simple but effective, the weight of the earth above held up by carefully chosen stone.