It is not. A single stone lintel, sitting about half a metre below the interior ground level of the enclosure, marks the entrance to something far older and more deliberate: a souterrain, a narrow underground passage built without mortar, its stones carefully laid against one another in what is known as drystone construction.

A cashel is a type of early medieval stone-walled enclosure, typically circular, that once defined a farmstead or small settlement. Souterrains were commonly built within or alongside such enclosures across Ireland, serving variously as cool storage spaces, places of refuge, or escape routes. The one at Rabaun is modest by any measure: the passage is roughly 55 centimetres wide and only 40 centimetres high, too tight for anything but a crawl. The depression above it is approximately three metres across and a metre deep, following the line of the cashel's enclosing wall. Centuries of field clearance have partially filled the passage with loose debris, which means its full extent is difficult to determine from the surface, though the lintel stone in the north-eastern side of the pit remains visible, an unambiguous sign that this collapse was not natural.