No hollow in the ground, no crop mark, no depression to catch the eye of a passing walker. The structure simply exists beneath ordinary farmland, invisible and largely unannounced.

A souterrain is an underground passage or chamber system, typically constructed during the early medieval period in Ireland, and thought to have served as a place of refuge, food storage, or both. The example at Corran was recorded by McCarthy in 1975 and comprises four chambers, their arrangement revealing a deliberate and somewhat varied design. The walls and floors are cut directly into rock to a depth of around one metre, with the roofs cut from earth rather than stone. Chambers one, three, and four incorporate some stone walling, while chamber two is notably more compact, roughly circular in plan with a diameter of two metres and a height of just over a metre. Chamber one is the most spacious, at three and a half metres long, two and a half metres wide, and two metres high, tall enough to stand in. Chambers three and four are progressively lower and narrower, with chamber four reaching only 0.8 metres in height, a tight crawl by any measure. The variation in scale across the four chambers is characteristic of souterrains generally, where a more accessible primary space sometimes gives way to deliberately constricted passages, possibly as a defensive feature intended to slow any intruder.