Local tradition and early Ordnance Survey records both pointed to the existence of a souterrain at Lisbeg, known in Irish as An Lios Beag. A souterrain is an underground stone-lined passage, usually associated with an early medieval ringfort and used variously for storage, refuge, or concealment. When the circular earthwork rath at this location was levelled around 1964, however, no such underground structure came to light.

What an earlier observer named Curran had recorded at the site was something slightly different: a roofed well fed by a spring that never ran dry. His description is oddly compelling. The structure was, he noted, well-built but so low that anyone wishing to enter had to get down on hands and knees to crawl inside. Whether this cramped, roofed chamber was in fact the same feature that local people remembered as a souterrain, or whether it was something altogether separate, has never been resolved. The rath itself, a ringfort of the circular type common across early medieval Ireland, was entirely gone before any systematic investigation could settle the question. J. Cuppage included the site in the 1986 Corca Dhuibhne archaeological survey of the Dingle Peninsula, and the ambiguity was duly noted there without resolution.