This is a souterrain, a type of man-made underground passage or chamber associated with early medieval ringforts across Ireland, most likely used for food storage, refuge, or both. What makes the Ballyanly example quietly interesting is not just its survival but its layout: two parallel earth-cut rooms, their long axes running north to south, each with a barrel-vaulted ceiling formed from the surrounding clay and stone, the whole thing tucked inside a ringfort that is still present above ground.

The souterrain was discovered in 1976 and investigated the following year by J.P. McCarthy, whose findings were published in 1977. The two chambers are closely matched in size: the first runs 3.5 metres long, one metre wide, and just 0.8 metres high, and connects at its northern end via a creepway to a second chamber of nearly identical dimensions, at 3.4 metres long and 1.1 metres wide. The present entrance lies at the southern end of the first chamber, though the original entrance has never been identified. Local knowledge adds a further detail: a second souterrain is said to lie to the west of the ringfort, though that one has since been closed up. Whether it was sealed deliberately or simply collapsed is not recorded.