The blocked entrance is the only visible sign of a souterrain, an underground stone-built passage or chamber constructed in early medieval Ireland, typically used for storage, refuge, or both. What makes this one quietly arresting is what can still be seen through the opening: a single chamber roughly 4.9 metres long and just over a metre wide, its walls built from stone and part of its floor apparently cut directly into the bedrock beneath.

The structure was discovered around 1981, which is relatively recent for a feature of this type. Souterrains in Ireland generally date from the early medieval period, roughly the sixth to the twelfth centuries, and are often found in association with ringforts or other settlement sites. The decision to block the Ratooragh example with a stone slab rather than excavate or preserve it more formally is not unusual; many souterrains across the country remain in similar states, known but not fully investigated, sitting at the edge of agricultural land that has been worked around them for generations. The rock-cut floor is a detail worth noting, since it suggests the builders took advantage of the natural geology rather than constructing everything from scratch, a practical choice that also tends to make these structures more durable over time.