A long, shallow trench runs across the western half of the enclosure, its edges soft with turf, its floor thick with blackthorn scrub. It does not look like much until you understand what it marks: the collapsed roof of an underground passage, a souterrain, now open to the sky after centuries of slow subsidence.

A souterrain is a man-made underground tunnel or chamber, typically cut into subsoil or built from stone and roofed with large flat slabs. They are found across early medieval Ireland, usually associated with ringforts or cashels, the latter being stone-walled enclosures that served as defended farmsteads. Their precise function is still debated, but they are generally thought to have been used for cold storage, refuge, or both. At Coolnaha, the passage once ran for at least seven metres, extending from the cashel wall towards the interior of the enclosure. The collapse has left a depression roughly two and a half metres wide and half a metre deep, running from west-southwest to east-northeast. At the western end, a single roof lintel remains in place, still sitting where the original builders set it. At the eastern end, the passage terminates at a heap of large stones, the tumbled remnants of what was once its furthest extent or a deliberate blocking.

What survives is fragmentary but legible. The in situ lintel is the most telling detail, a flat capstone that never shifted even as the rest of the structure gradually gave way around it. The blackthorn colonising the base of the depression has, in its own way, sealed the feature from further casual disturbance, making the trench look more like a natural hollow than the engineered void it once was.