It measures roughly 3.3 metres north to south and 1.2 metres east to west, and it sits only about 20 centimetres below the present surface, which is shallow enough that it might easily be passed over as a natural depression. At its southern end, a stone lintel remains in place, marking what was the entrance passage to the structure beneath.

This is a souterrain, a type of underground chamber built during the early medieval period in Ireland, typically from around the seventh to the twelfth century. The word comes from the French for underground passage, and these structures were constructed from stone, sometimes corbelled, sometimes roofed with large flat lintels of the kind that survives here. Archaeologists have interpreted them variously as places of refuge, cool stores for dairy produce, or both at different times of year. What makes the Gortnalibbert example quietly puzzling is the absence of any associated enclosure. Most souterrains in Ireland are found within or close to a ringfort, a rath, or some other kind of enclosed settlement, where the underground space served the community living above it. Here, there is no evidence that any such enclosure ever existed, which raises the question of what the souterrain was attached to, and why nothing of it has survived or been identified.