This is a souterrain, an underground structure of the early medieval period, built without mortar from dry-stacked stone and roofed with flat slabs. They appear in their hundreds across Ireland, typically associated with ringforts, and are thought to have served as places of refuge, food storage, or both. The one at Tullyglass is modest but well preserved in its dimensions: the larger chamber runs to just under four metres in length and barely over a metre wide, with a ceiling low enough that no adult could stand upright inside it.

The structure was recorded in 1933 and published by Ó Riordain in 1935, with further measurements provided by McCarthy in 1977. Those figures give a reasonably clear picture of what lies underground: two rectangular chambers joined by what is called a creepway, a deliberately narrow connecting passage that would have slowed any intruder and forced them to move through on hands and knees. The dry stone walls and flagged roof are characteristic of the type, built to last through careful fitting of stone rather than any binding material. That this one has survived with its proportions intact and on record is itself notable, since many souterrains across the country have collapsed or been disturbed over the centuries.