The only sign of what is underneath is a filled-in hollow roughly a metre long and a metre wide, an unremarkable depression in the grass that happens to mark the entrance to a souterrain, an underground stone-lined tunnel or chamber of the kind built across early medieval Ireland, most commonly between the seventh and twelfth centuries.

Souterrains were typically constructed in association with ringforts or other early settlement sites, and served a range of purposes that archaeologists still debate, including food storage, refuge, or simply secure space beneath a farmstead. They were usually built by digging a trench, lining it with drystone walling, roofing it with large flat slabs, and then covering the whole structure with earth. The entrance at Tooms has long since been infilled, leaving only its approximate dimensions on record. Without excavation, the extent of any passage or chambers below remains unknown.