This is a souterrain, an underground stone-built passage constructed in the early medieval period, typically associated with a nearby rath or ringfort. Souterrains are found across Ireland in considerable numbers, and their precise function is still debated; theories range from cold storage for dairy produce to places of refuge in times of attack. The one at Drumgolat is a particularly compact example, and what makes it notable is its internal arrangement, a structure within a structure, folded tight into the earth.

According to a description recorded by McCormick in 1978, the souterrain consists of an entrance passage running northward, roughly two metres long and about a metre wide, with drystone walls and a lintelled roof, meaning flat stone slabs laid across the top rather than an arch. The passage is entered from the south and stands between 0.4 and one metre high. At its northern end, a narrow creep, just 0.7 metres long and 0.4 metres wide and covered by a single lintel, connects to a slightly larger inner chamber. That inner chamber runs approximately 2.3 metres north to south and widens from 0.45 metres at its northern end to around 0.9 metres at the south, with headroom increasing from half a metre to 1.2 metres. The northern end of this inner chamber has sustained some damage. The souterrain sits just north of the centre of a rath, and it leads northward from a rectangular mound or hut site nearby, suggesting the underground structure was one component of a small but organised early medieval settlement complex.