It is not caves, exactly, but the label is understandable. What lies beneath the grass at Carrowcastle is a souterrain, an underground passage built in drystone construction, of the kind used throughout early medieval Ireland for storage, refuge, or both. The fact that cartographers more than a century ago reached for a different word altogether suggests the structure had already lost its original context in local memory, surviving instead as an unexplained hollow in the ground.

The souterrain sits within a rath, a circular enclosure defined by an earthen bank and ditch, which in early medieval Ireland typically served as a farmstead. Inside the rath, the western half of the interior sits at a noticeably higher level than the eastern half, and it is here that two lintelled openings give access to the underground passages. One opening, near the centre of that raised western area, is preceded by a shallow, grass-covered depression with stone-faced sides, representing a short section of passage that has lost its roof cover. A flat stone lintel still sits in place on the southern side of this depression, covering an entrance into an intact passage that runs southward. That opening measures roughly 87 centimetres wide and only 35 centimetres high, with drystone walls of rounded stones, the kind of tight, low access that would have made uninvited entry distinctly awkward. A second opening, wider and taller at 1.23 metres by 0.65 metres, sits at the inner edge of the bank on the western side and covers a drystone passage extending to the south-southeast for several metres. A slight dip in the ground about four metres to the south-southwest may indicate a further, as yet unexcavated, section of the system.

The two openings are visible at the surface, and the partial roofless section can be seen without entering, though the low lintels make clear that these passages were never built for comfort. The western half of the rath interior, being the higher ground, is the area to focus on when trying to locate them.