This is a souterrain, an underground passage constructed from carefully laid drystone walling, of a type built throughout early medieval Ireland, most likely as a place of refuge or cool storage. The one at Derrineden is largely collapsed now, but its mouth still sits just 1.3 metres south-west of one of the hut structures it once served, a relationship that hints at the domestic life arranged around it long ago.

The passage itself, though no longer fully navigable, retains a legible shape. It runs northward for roughly three metres before making a turn to the east-south-east for a further two metres. At that point, fallen roofing lintels, the large flat stones that would once have formed the ceiling of the passage, have come down and block any further entry. The structure was recorded and described by A. O'Sullivan and J. Sheehan in their archaeological survey of the Iveragh Peninsula, published by Cork University Press in 1996, which catalogued the remarkable concentration of early settlement remains across this part of Kerry. The site sits within a broader complex that includes the hut foundations nearby, suggesting a small but organised settlement whose inhabitants went to considerable effort to construct this underground annex beneath their feet.