The tradition is specific enough to be credible: a passage built from drystone walls and roofed with flat lintels, running beneath the earth in the manner of a souterrain. These underground stone-lined chambers and corridors were constructed throughout early medieval Ireland, typically within or beside a rath, and are thought to have served as places of refuge, food storage, or both. At Brackloon, however, the structure has left no mark on the surface whatsoever, and dense overgrowth inside the enclosure has made any physical inspection difficult.

The souterrain is associated with a rath, the remains of a roughly circular earthen enclosure of the kind that once served as a farmstead during the early medieval period, roughly the fifth to twelfth centuries. Raths are among the most common archaeological monuments in Ireland, and they frequently conceal souterrains that were deliberately dug into the interior ground and sealed. What survives at Brackloon is the rath itself, catalogued separately, and the persistent local knowledge that something lies beneath it. That oral tradition is doing considerable work here, holding the record of a structure that archaeology has not yet been able to confirm or describe in any detail.