The entrance, cut into the south-western face of the bank, measures less than eighty centimetres across and not quite half a metre high, which gives an immediate indication of how the structure was designed to be used: not comfortably, and not by anyone in a hurry.

A souterrain is an underground stone-built passage or chamber, typically associated with early medieval ringforts in Ireland, where they served as places of refuge, storage, or concealment. The example at Ráth Ciaráin is unusually elaborate. Its five passages, aligned predominantly north to south after the first, vary in height up to 1.9 metres and are constructed using slightly corbelled drystone walls, meaning the stones angle inward slightly as they rise, lending the structure a degree of stability without mortar. Each passage is separated from the next by deliberate obstacles: a large hanging slab that drops to within sixty centimetres of the floor between the first and second passages, and a low lintelled creepway only sixty centimetres high giving access to the third. These were not accidental features. Anyone moving through the souterrain under duress would know the layout; a stranger would not. A narrow shaft in the west wall of the second passage, positioned at roof level, was most likely an air-vent, suggesting the space was intended for occupation over some duration. The first passage also contains a small blocked opening in its south-western wall, roughly 1.2 metres from the present entrance, which may represent the souterrain's original point of access. A large perforated slab sitting at the centre of the enclosure above ground appears to overlie the fifth and now-collapsed section of the passages, and may once have served as a second entry point from within the fort itself.