Beneath the road connecting the Midleton-to-Youghal route with the Midleton-to-Dongourney road, a stone-built souterrain had been quietly waiting. A souterrain is an underground passage or chamber, typically constructed during the early medieval period in Ireland, built from drystone walling and roofed with large flat stones called lintels. They are thought to have served as places of refuge, storage, or both, and are found across the country, often beneath or beside ringforts.

The passage at Killeagh runs east to west for just over seven metres, though it is narrow, less than a metre wide, and low enough that it could only have been entered by crawling, its height ranging from roughly half a metre to just under a metre. The subsidence that revealed it was caused by the collapse of one of the roof lintels near the centre of the passage. Investigation in 2002 found that the western end was a dead end, while the eastern end had already suffered a more substantial collapse, possibly the result of a deep roadside drain having been dug at some point in the past, cutting through or destabilising that section. With further investigation blocked, the displaced capstone was removed, precast concrete lintels were installed over the opening, a concrete slab was cast in place on top, and the road was reinstated. The souterrain now lies sealed once more, a few centimetres below the ordinary surface of a back road in east Cork.