The souterrain at Cill Mhic An Domhnaigh is, in the most literal sense, a place that cannot currently be found in full, even by those who go looking for it.

The wider monument is a univallate rath, a type of circular enclosure defended by a single earthen bank and accompanying fosse, or ditch, that was a common form of defended farmstead in early medieval Ireland. This one sits on a gently east-facing slope and retains traces of three or four hut sites within its interior. The souterrain, an underground stone-lined passage typically used for storage or refuge, opens off the northwestern side of the westernmost hut. A sketch plan drawn up in 1968 by the Office of Public Works recorded a passage running southwest from that entrance, apparently terminating at a second opening in the outer face of the ringfort bank. By the time the Dingle Peninsula archaeological survey was published in 1986, compiled by J. Cuppage for Oidhreacht Chorca Dhuibhne, that southwestern area had become so densely overgrown that the precise location of the exit point could not be identified. It remains unlocated.

The passage itself is no longer accessible from the interior either, so what survives above ground is essentially a well-preserved earthwork with a hidden interior, its underground corridor traceable only on paper. The 1968 sketch plan is, for now, the clearest picture anyone has of where the souterrain goes and where it ends.