A souterrain, as these underground structures are known, is a type of dry-stone tunnel or chamber found across early medieval Ireland, typically associated with nearby settlements and thought to have served as cold storage, refuge, or both. What makes the one at Grange quietly compelling is the particular logic of its construction: a wedge-shaped first chamber that narrows deliberately at one end, forcing anyone moving through it to compress themselves into a creepway barely half a metre wide before reaching a second chamber beyond.

The site came to light in 1963 and was subsequently investigated by M. J. O'Kelly of University College Cork, one of the most significant Irish archaeologists of the twentieth century, best known for his excavations at Newgrange. The structural detail recorded here draws on McCarthy's 1977 account. Chamber one runs roughly northeast to southwest, measuring three metres in length, between 0.8 and 1.3 metres wide, and 1.3 metres high. The side walls are corbelled, meaning the stones are laid so that each course projects slightly inward over the one below, creating a self-supporting narrowing without the need for a full arch. The roof was formed from flat lintels, one of which has since collapsed. At the southwest end, the narrower terminus, a short stone-lined creepway connects to a second chamber, though that second space is now inaccessible. The first chamber can still be entered through the gap left by the fallen roof stone, which offers an accidental window into the structure that its original builders almost certainly never intended.