The gap beneath one of the entrance roof slabs is simply too narrow to squeeze through, leaving the interior sealed not by a locked door or a conservation order, but by the accumulated weight of its own stonework. A souterrain, for those unfamiliar with the term, is a man-made underground passage or chamber, typically associated with early medieval settlements in Ireland, and thought to have served as a place of refuge, storage, or both. The one at Kilteenbane, tucked between the foot of Corrin mountain to the west and the Finglas river to the east, is inaccessible and yet surprisingly well documented.

Survey work, published in J. Cuppage's 1986 archaeological survey of the Corca Dhuibhne region, recorded a structure of some complexity. The main passage runs roughly north-north-east for approximately five metres, built entirely in drystone construction and roofed with large flat slabs. Off the east wall of this first passage, about a metre from the entrance, a second passage branches away and extends around twelve metres towards a clochaun, a small drystone hut or beehive cell, nearby. Roughly 1.75 metres along that second passage there is a porthole slab, a round-headed stone with an opening cut through it, which would have forced anyone moving through the tunnel to slow down and squeeze past, an obvious defensive feature. A third passage once branched off to the north-east before surfacing. The current entrance on the western edge of the platform is almost certainly not the original one; the builders would have placed the entrance within the thickness of the surrounding bank, concealing it from outside view. A drystone air-shaft running from the passage wall up to the outer face of that bank suggests careful planning, not improvisation. The whole structure sits within a rath, a ringfort enclosed by an earthen bank, and the traces of three hut sites within the same enclosure indicate a settlement of some substance once occupied this ground.