This is a souterrain, an underground tunnel or chamber of the kind built throughout early medieval Ireland, typically used for food storage, refuge, or both. What makes Tullycommon quietly arresting is the layering of structures around it: the souterrain does not sit in isolation but within a cashel, a roughly rectangular stone-walled enclosure, which is itself set inside a large field system that once organised the land across this windswept upland.

The antiquarian Thomas Johnson Westropp recorded the site in 1905, noting that the passage appeared to extend beneath a large rectangular slab. That detail alone conjures something of the original construction, a deliberate stone-roofed corridor driven into the ground beneath what was already a settled and bounded landscape. The souterrain leads from the northern side of a house site within the cashel, suggesting that whoever lived there had direct access to the underground space, a common arrangement in early Irish settlement where the dwelling and its subterranean annex formed a single functional unit. The passage has since collapsed, leaving what is now described as a sunken cave-like depression rather than an accessible tunnel.