A souterrain, the term for an underground stone-lined tunnel or chamber typically built during the early medieval period in Ireland, these structures are thought to have served as places of refuge or storage, and they were carefully constructed to last. This one apparently did not, at least not indefinitely. Around 1976, the ground above it collapsed, and rather than excavate or preserve what lay beneath, someone simply backfilled the hollow and moved on.

The earliest record of the souterrain comes from a 1937 account by Broker, who noted that flags, meaning flat stone slabs, had been taken up and a lined passage found beneath. That brief description is almost all that survives of what was presumably a deliberate investigation, one that confirmed the presence of an intact or partially intact underground structure within the bounds of an existing ringfort. Ringforts, circular enclosures defined by earthen banks or stone walls, are among the most common archaeological monuments in the Irish landscape, and it was not unusual for their builders to include a souterrain as part of the original design. The two features at Annagloor share the same site, suggesting they may well be contemporaneous, both belonging to an early medieval farmstead whose surface traces are now all that remain visible.