It is not a natural cave at all, but a souterrain, an artificial underground passage and chamber system built from drystone walling and roofed with flat lintels. Souterrains are found across early medieval Ireland, typically associated with enclosed settlement sites, and were used variously for storage, refuge, or both. What makes the Farranyharpy example quietly arresting is where it sits and what survives inside it.

The souterrain lies within a cashel, a type of early stone-walled enclosure that served as a defended farmstead or settlement. The entrance is cut into the inner face of the cashel's ruined enclosing wall on the southwest side. From there, a passage leads into a first chamber, which connects eastward through a low lintelled creepway, the kind of constricted crawl-through passage that would force anyone entering to slow down considerably, into a second chamber. That second chamber contains something rather particular: a small stone-lined cupboard built into its eastern wall, a detail that speaks to deliberate domestic or practical organisation underground. A further creepway opens off the north wall of the second chamber but is now blocked, though it appears to continue northward. Above ground, the line of the whole structure is still legible as a shallow linear depression running roughly seven metres across the cashel interior, a faint crease in the turf tracing the buried architecture beneath. A sketch plan made in 1986 captured the layout before conditions changed further, preserving a record of a structure that most people walking the site might not know was underfoot at all.