A souterrain, to give the term its context, is an underground stone-lined passage or chamber, typically associated with early medieval ringforts and used for storage, refuge, or both. The one recorded at Ballygeagoge was noted on the Cassini edition of the Ordnance Survey six-inch map as a small circular opening in the western quadrant of a nearby ringfort, but it was absent from the 1840 edition of the same survey, absent again from the 1897 twenty-five-inch edition, and by the time satellite imagery was captured between 2011 and 2013, no surface remains were visible at all.

The most detailed description on record comes from a 1931 account by O'Shaughnessy and Carroll, who described the broader site as comprising two moats or lisses with souterrains. A liss, or lis, is another name for a ringfort, the circular earthwork enclosures that dot the Irish countryside in their thousands, and whose interiors often conceal these subterranean features. The souterrain at Ballygeagoge sits within a landscape unusually dense with early monuments. The enclosure known as Cahir Mortle lies roughly 500 metres to the north-west, accompanied by a further ringfort. A stone circle lies just 264 metres to the east. The site itself is positioned about 40 metres west of the townland boundary with Ballinlyna Lower, a precise detail that points to how carefully these monuments have been logged, even when the monuments themselves have become all but untraceable.

The site lies in pasture and there is nothing on the surface to mark it out. For anyone exploring this part of Limerick, the surrounding monuments offer more visible rewards, and cross-referencing the Cassini-edition Ordnance Survey map against current landscape features is the most reliable way to orient yourself in the area. The value here is less in what you can see than in what the cartographic and documentary record suggests once existed, and in the quiet accumulation of early medieval activity across this particular hillside.