The operative word is "possible": the site at Coomlettra is recorded not as a confirmed structure but as a candidate, a place where the evidence was suggestive enough to note but not conclusive enough to settle the question.

A souterrain is an underground passage or chamber, typically dry-walled and roofed with stone lintels, built during the early medieval period in Ireland as a place of refuge or storage, often associated with a nearby ringfort or settlement. The Coomlettra example sits in the landscape between Lough Caragh and Lough Acoose on the Iveragh Peninsula, documented in the 1996 archaeological survey of South Kerry compiled by A. O'Sullivan and J. Sheehan. That survey recorded hundreds of sites across one of Ireland's most archaeologically layered landscapes, and even among that company this one stands out for its near-total absence of visible evidence. Whatever may lie underground here, the surface gives nothing away.