This is a souterrain, an artificial underground structure of the early medieval period typically cut or built beneath a rath, the circular earthwork enclosure that served as a farmstead. The Russagh example sits in the north-western quadrant of just such a rath, and its survival, partial as it is, gives an unusually detailed picture of how these subterranean spaces were constructed and used.

The souterrain was recorded in 1992, though the means of entry that year was not an original one. At some earlier point, an unrecorded cutting had been made into the site, dropping roughly 2.7 metres below the surface of the rath to reach the first chamber. That intervention may have destroyed one or more chambers entirely. What remained was still complex: three interconnected chambers, each roughly rectangular, linked by creepways, the short crawl-through passages that allowed passage while making the space defensible or at least difficult to rush through. The first chamber, measuring about 4.4 metres north to south and just under 1.5 metres wide, has a stone-slabbed floor and a small alcove set into its north-western corner. A second creepway leads south into chamber two, which is slightly irregular, widening and rising at its north-eastern end. This chamber has its own alcove, a narrow angled opening in one wall, and a small hole in the roof covered by a loose slab, possibly a ventilation point or a means of communication with the surface. Chamber three, the smallest, narrows to a wedge where its floor rises to meet the roof, and retains a blocked-up construction shaft, the vertical opening through which spoil would have been removed during the original digging. By 1999, the cutting made before the 1992 survey was still visible at the inner edge of the rath's bank, a reminder that the site had been disturbed well before any formal record was made.