This is the souterrain, a type of drystone-built underground chamber commonly associated with early medieval ringforts, used variously for refuge, storage, and ventilation. At Kilteenbane, the entrance has shifted over time to a point at the western edge of the enclosure platform, though this is almost certainly not where people once entered; the original opening would have been buried within the bank itself. Today the gap beneath the roof slabs is too narrow to admit a person, so the interior geometry described by earlier visitors remains, for now, a matter of historical record rather than direct observation.

The site sits on a level shelf of ground between the foot of Corrin mountain to the west and the Finglas river to the east, where the land drops sharply toward the water. When Browne, Armstrong, and Macalister examined it in 1911, two enclosing banks were still discernible, though even then the outer one was described as much ruined. More than a century later, that outer bank survives only in the south-eastern quadrant, reaching 1.9 metres high on its exterior face. The inner bank has largely been levelled inward, leaving what is now a circular platform, slightly sunken at the centre, with a 2.3-metre-wide entrance on the eastern side, one edge of which is lined with horizontally-laid stone slabs. Unusually, there is no matching gap in the outer bank to align with this entrance, which suggests either a deliberate design choice or significant later disturbance. Inside the enclosure, three clochans, which are small dry-stone corbelled huts, were recorded; one, at the south-south-east, retains a portion of its inner wall-face to a height of about 0.6 metres with a slight inward corbelling. A second is now little more than scattered loose stones, and a third, in the northern sector, is so thoroughly choked by vegetation that only a short stretch of its outer wall remains visible.