The ringfort, or rath, a circular enclosure of the kind typically associated with early medieval farmsteads in Ireland, was already being eaten away by the time anyone thought to document it properly. A road pushed through in a northwest to southeast direction at some point after the 1841 to 1842 Ordnance Survey mapping, which had recorded the site as a complete circular enclosure. By the time archaeologists reached it, the earthen bank, which had once run from the northwest through west to southwest and stood roughly 0.4 metres above the surrounding ground, had been completely levelled. What remains is the souterrain hidden beneath the southwestern interior.

A souterrain is an underground stone-built passage or series of chambers, typically associated with early medieval raths and thought to have served for storage, refuge, or both. The one at Rathea is a particularly well-preserved example. Constructed from drystone walling and capped with large lintel stones, it consists of two chambers connected by narrow creep passages, those low, tight tunnels, usually just wide enough to admit a crawling adult, that are a characteristic feature of souterrain design. The first chamber is roughly circular in plan and rises to 1.5 metres, built using the beehive technique in which courses of stone are corbelled inward to a central capstone, here a flat slab 0.8 metres across. A creep only 0.5 metres wide and 0.4 metres high connects it to a second, D-shaped chamber that opens to a height of 1.68 metres. A further passage runs from the southeast side of that chamber for around 1.62 metres before collapsing. The full complexity of the underground network documented by C. Toal in the North Kerry Archaeological Survey of 1995 suggests a structure that was carefully engineered rather than improvised, even if the rath that sheltered it above ground is now almost entirely gone.