The entrance, measuring just 0.8 metres by 0.5 metres, sits close to the centre of a caher, the Irish term for a stone-walled circular enclosure, and what lies beyond it is a souterrain: an underground passage or chamber system built from drystone masonry, typically associated with early medieval settlement in Ireland and thought to have served as storage space, a place of refuge, or both.

The souterrain at Garrough is more elaborate than its modest entrance suggests. The first passage runs northeast to southwest for 3.5 metres, its roughly coursed drystone walls rising to just 0.5 metres in height and roofed with large flat slabs. At the northeast end, a small semicircular chamber opens from the southeast side-wall, roofed with three lintels and measuring 1 metre wide, 0.6 metres high, and 1.3 metres deep. The southwest end of that first passage then connects to a second, longer passage running northwest to southeast for 5.2 metres, though a rise in the floor level seals off its northwest end. The overall layout, two intersecting passages with a side chamber, is characteristic of the more complex end of souterrain construction, where builders clearly invested considerable effort in creating a functional and durable underground space within the caher's protective boundary.