That alone is worth pausing over. Souterrains, the underground stone-lined tunnels built during the early medieval period in Ireland, typically run in relatively straight lines, used for cold storage or as refuges in times of trouble. The one at Treankeel is different: it changes direction twice, tracing a rough zig-zag across the western half of the enclosure it sits within, and that deliberate angularity suggests a builder with specific intentions, whether for structural reasons or to complicate entry for anyone unwelcome.

The souterrain lies within a rath, the familiar circular earthwork enclosure of early medieval Ireland, typically defined by a bank and ditch and associated with a farmstead. At Treankeel, the souterrain's route can still be read from the surface as a grassed-over linear depression, roughly two metres wide and between 0.4 and 0.6 metres deep. It runs from a point near the rath's northern scarp for 7.5 metres to the south-east, then turns south-west for another 4.7 metres, before continuing south-south-east for a final 11 metres. The total underground distance covered is considerable. At the north-north-west end, a large stone slab is visible in a shallow sub-surface void, most likely an original lintel or roof slab still sitting more or less where it was placed, probably more than a thousand years ago.