The souterrain, a type of artificially constructed underground chamber or tunnel commonly associated with early medieval Irish settlements, curves slightly as it runs from west-southwest to east-northeast, stretching around four metres in length. That modest measurement, however, likely undersells what was once there.

The passage is drystone built, meaning its walls and roof were constructed without mortar, from carefully placed stone. Access today comes through a breach at the western end, and the presence of additional drystone walling at that point suggests the souterrain once continued further in that direction before it was either collapsed or lost. Souterrains were typically used for storage, refuge, or both, and they are frequently found within ringforts, the circular enclosed farmsteads that were the dominant settlement form in early medieval Ireland. The ringfort here at Knockereen is described as probable rather than confirmed, which is itself a reminder of how much of the Irish landscape still sits somewhere between identification and certainty.