The souterrain at Cabraghkeel is accessible through an opening on the north-north-east side of the surrounding bank, where a drystone-walled corridor, roofed with large stone slabs, leads inward for a short distance before it becomes completely blocked. What lies beyond the obstruction is unknown, though the ground nearby offers a possible clue.

A souterrain is an underground stone-built passage or chamber, typically constructed during the early medieval period in Ireland and associated with ringforts, known in Irish as raths. These structures served various purposes, most likely storage and refuge. The Cabraghkeel example sits within its rath, and a short distance north of the fort's interior centre, the ground has given way into a roughly rectangular hollow measuring approximately six metres east to west, 2.4 metres north to south, and around 0.6 metres deep. This depression is thought to mark the location of a collapsed chamber, the kind of underground room that would originally have been reached from the passage. The pattern is familiar in Irish souterrain archaeology: a linear entry passage connecting to one or more chambers, the whole system cut into the earth and lined with carefully laid stone. At Cabraghkeel, time and settling soil appear to have brought the chamber roof down, leaving only that shallow, tell-tale hollow in the grass.