Beneath or within it, there are indications of a souterrain, one of those underground stone-lined passages or chambers built in early medieval Ireland, typically for food storage, refuge, or as an escape route from a nearby ringfort or enclosure. The catch is that no one has been inside it in living memory, and access to the site has not been granted to researchers.

The enclosure itself is not entirely invisible to history. It appears on both the 1842 and 1897 Ordnance Survey maps, meaning it was a recognisable feature of the landscape throughout the nineteenth century. By 1940, when Kerry Field Notes recorded the site, the souterrain was noted as present but not open, suggesting the entrance was blocked or collapsed even then. C. Toal's North Kerry Archaeological Survey, published in 1995, drew on that earlier observation without being able to add much to it, since permission to visit the site was not obtained. The enclosure sits in close proximity to Caherfearta, a cashel or stone-built ringfort nearby, and the pairing of an enclosure with a souterrain is a pattern well known from early medieval settlement across Munster.

What makes the Killeacle souterrain quietly compelling is precisely its resistance to documentation. It has been noted, mapped, and named, but never properly examined. The underground structure, if intact, remains sealed, its contents and construction unknown.