The structure is a souterrain, an underground passage or chamber built from drystone walling, a technique in which stones are laid without mortar, and these were constructed throughout early medieval Ireland as places of refuge, storage, or concealment, most often within or alongside a rath.

A rath, sometimes called a ringfort, is a roughly circular enclosure defined by an earthen bank and ditch, and this one at Rabaun still contains the souterrain within its south-western quadrant. The main passage runs on a north-east to south-west axis for approximately ten metres, beginning near the rath bank and ending close to the centre of the enclosure. Near the south-western end, a section roughly two metres long remains structurally intact, with drystone walls still carrying their original roof lintels. Beyond that, the passage has partly collapsed, and can be followed only as a shallow, sod-covered channel, around sixty centimetres deep, tracing the line of the tunnel below. Midway along its length, a side passage branches off, running two metres to the north-west before turning north-north-east for a further two metres, its course also legible at ground level as a faint depression in the grass. At the north-eastern end of the main passage, where it reaches the interior of the rath, the tunnel meets a circular stone feature whose purpose remains uncertain: it may be an additional chamber forming part of the souterrain system, or it may be the remains of a house that occupied the same space at some point in the site's history. That ambiguity is itself telling, a reminder of how densely layered the activity within even a modest enclosed settlement could be, and how much of it has simply settled back into the ground.