A souterrain is an underground stone-lined passage, typically built during the early medieval period and associated with ringforts or cashels, the latter being the stone-walled enclosures that dot the Irish landscape. The one recorded at Killeeneen Beg in County Galway sits within the southern half of just such a cashel, and by the time anyone looked for it carefully, the vegetation had won. No visible trace remained.

What survives is a paper trail rather than a physical one. Writing in 1916, a Redington described what was still visible then as the mouth of a souterrain passage, built from large stones, suggesting an entrance to an underground chamber or corridor that would once have served as storage, refuge, or both. McCaffrey noted it again in 1952, by which point it had presumably not yet vanished entirely from view. At some point between these references and a later inspection, the site became completely overgrown and obscured. The cashel itself, a separate monument, still has a record, but the souterrain it once contained has effectively been reclaimed by the ground around it.