The structure measures three metres long, one and a half metres wide, and just under one and a half metres high, roofed by seven capstones and oriented along an east-west axis. It is, in other words, precisely large enough for a person to enter but not to stand upright, which is more or less the point.

This type of structure is known as a souterrain, an underground stone-lined passage or chamber associated with early medieval ringforts across Ireland. Ringforts, which were enclosed farmsteads typically dating from roughly the sixth to the tenth centuries, often incorporated souterrains as places of refuge, storage, or both. The Knockalassa example is described as well constructed, its seven capstones still doing their job after well over a thousand years. At the western end, a creepway, the narrow connecting passage that linked one chamber to the next, has collapsed, though traces at ground level suggest it once led to at least one further chamber beyond. What that further space contained, or how extensive the full souterrain complex may have been, remains unclear.