These connecting passages are known as drop-hole creeps, and negotiating them would have required a person to drop down and crawl through on hands and knees, a feature that made the interior easy to defend and difficult to stumble into by accident. The structure is a souterrain, an underground chamber or series of chambers built from drystone masonry without mortar, a type of construction common in early medieval Ireland and typically associated with ringforts. What makes the Lecarrownagappoge example notable is how well it has survived.

The souterrain sits within the western sector of a multivallate rath, meaning a ringfort enclosed by multiple earthen banks and ditches rather than a single ring. The underground complex runs in three distinct directions: the first chamber tracks roughly east-northeast to west-southwest, the second turns to run northwest-southeast, and the third heads east-west, giving the whole structure a loose zigzag beneath the ground. Each chamber widens slightly towards its centre and closes with a curved end-wall, details that suggest careful, deliberate construction rather than improvised digging. The first chamber measures nine metres in length, the second just over seven, and the third nearly seven, with ceiling heights between roughly 1.5 and 1.7 metres throughout. Scholars including Knox and Redington noted the site as early as 1916, and it has been referenced in a handful of subsequent studies, including work by Westropp in 1919 and Ní Chinnéide in 1978. At the western end of the third chamber there is evidence of digging, and the southern wall shows signs of interference, suggesting the site attracted attention, whether from curious explorers or earlier antiquarians, long before any formal recording took place.