That researcher, Hartnett, noted a diameter of approximately eighty feet, a figure that matches reasonably well with the roughly twenty-five metre enclosure shown on the Ordnance Survey six-inch map of 1842, where it appears as a hachured circle, the conventional cartographic shorthand for a raised or embanked earthwork. What the map also records, and what makes this particular site quietly interesting, is the presence of a souterrain inside the eastern bank. A souterrain is an underground stone-lined passage or chamber, typically associated with early medieval ringforts and thought to have served for storage, refuge, or both. That one survived long enough to be mapped suggests the site was once a substantial enough enclosure to warrant the effort of constructing it.

The ringfort itself belongs to a class of monument known as a rath, a roughly circular enclosure defined by an earthen bank and ditch, which served as a farmstead during the early medieval period in Ireland, broadly the fifth to twelfth centuries. The eastern bank at Knockglass was also the location of a lime kiln, a later insertion almost certainly dating from post-medieval agricultural activity, when landowners routinely quarried or repurposed earthworks for building material and field improvement. That combination of souterrain and lime kiln in the same bank neatly captures how these sites were used, modified, and ultimately consumed over centuries. Adding further depth to the immediate landscape is a wedge tomb sitting roughly a hundred and twenty metres to the west-southwest, a megalithic monument of a completely different era, predating the ringfort by perhaps three thousand years.