It appears on the Ordnance Survey six-inch map of 1902 under the matter-of-fact label "Cave", which is precisely how such features were often recorded by surveyors who encountered them as dark openings in the ground without any ready category to hand. Today there is no visible trace of it at all.

A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval settlement in Ireland, used for storage, refuge, or both. They are almost always found in close relation to a ringfort, the circular earthwork enclosure that was the standard farmstead of the early Irish countryside, roughly from the fifth to the twelfth century. The Derrygool example sits at the centre of one such enclosure, a positioning that is not unusual but does underline how integral these underground spaces were to the life of the settlement above. The 1902 OS mapping gives us a fixed point in time when the feature was still identifiable as a surface opening, which means the loss of any visible trace has occurred in the century or so since.