What the cartographers were gesturing at was a souterrain, an underground passage of the kind built by early medieval Irish farming communities, typically for storage or as a place of refuge. The label stuck across nearly a century of mapping, which says something about how the structure presented itself to people on the ground: not obviously as part of a ringfort, but as a mysterious hollow in a field.

The rath at Tullanacorra sits at the break of slope on the northern side of a ridge, a position that gives it wide views across lowlands stretching from north-west to north-east, while the rising ground to the south keeps it from being visible from that direction. The enclosure itself is a raised circular platform, roughly 38 to 39 metres across, defined by an earthen bank on part of its circuit and by a natural-looking scarp on the rest, dropping as much as two metres on the north side. A shallow depression at the base of the eastern bank may be the remnant of a fosse, the ditch that in many raths would have run around the full perimeter, though it cannot be traced further. Inside, the ground is level across the centre and western half, with a slight fall towards a dilapidated entrance gap on the north-east, about 2.5 metres wide, with a low ramp running outward from it. Near the centre of the interior, a low rectangular platform of roughly six by six and a half metres, outlined in sod-covered stone, may be the footprint of a house. An internal scarp runs eastward from its corner to the rath's own bank, dividing off a slightly lower section of ground in the south-east quadrant. The souterrain lies adjacent to the south-west end of this platform: a partially collapsed stone-lintelled passage, with intact stretches still accessible at either end. The whole rath is encircled by hawthorn trees, and it sits within a notably dense landscape of similar enclosures, with two further raths within 200 metres and a dispersed cluster spread across the surrounding half-kilometre.