The passage is blocked with debris, and the opening itself is all that remains visible of a souterrain, an artificially constructed underground tunnel or chamber, typically built during the early medieval period as a place of refuge, storage, or concealment, and usually associated with a nearby settlement or ringfort.

The rath, or ringfort, at Islandboy in Co. Kerry sits in a townland recorded in the Ordnance Survey Name Books under two placenames: Lismore, from the Irish Lios Mór meaning the great fort, and Lisbeg, from Lios Beag meaning the small fort. These names suggest the area was once associated with at least two distinct enclosures, and it is thought that this souterrain and a neighbouring site together represent the monuments the name books had in mind. The souterrain opening lies in the western sector of the rath's interior, its entrance measuring roughly 0.9 metres by 0.4 metres, a tight squeeze even were the debris cleared. How far the passage extends, or what form it takes underground, is no longer known from direct inspection.