Four separate depressions have opened in the surface of an enclosed earthwork, each no more than a metre across, and together they trace the line of something older running beneath: a souterrain, a type of man-made underground passage built during the early medieval period, typically associated with ringforts and used variously for storage, refuge, or both. The collapses are small enough to be easily overlooked, but peer into them and the structure beneath becomes visible, dry-stone walling carefully laid, and flat lintel stones still holding their position across the top of a passage roughly eighty centimetres wide.

The souterrain sits within a rath, the Irish term for a ringfort, one of the most common archaeological monument types in the country. These were enclosed farmsteads, their interiors ringed by earthen banks, and it was common practice to construct souterrains inside them. At Brackloon, the underground passage begins near the northwestern bank of the rath and runs southeast towards the interior, before turning to follow a more southerly direction. A large flat slab protruding from the ground near the north-northwestern bank may mark another point along its length, though its precise relationship to the passage is uncertain. The passage has not been excavated, and what the collapses reveal is therefore a rare, unmediated glimpse into a structure that has otherwise remained sealed for centuries.