One of those chambers no longer exists in any accessible sense, having collapsed around 1982 according to local memory. The other remains, modest in its dimensions but quietly significant as a piece of early Irish underground architecture.

The structure is a souterrain, a type of man-made underground passage and chamber system commonly associated with ringforts across Ireland, typically dating to the early medieval period. Their precise function is still debated, with theories ranging from cool storage for dairy produce to refuge in times of attack, and many sites suggest a combination of both uses. At Knocknageeha, the entrance opens into a central chamber measuring roughly 1.05 metres east to west and 1.2 metres north to south, with a depth of around one metre. From there, narrow creepways, passages low enough to require crawling, extend toward further chambers to the north and south. The southern creepway now leads nowhere useful, its destination having given way at some point in the early 1980s. The souterrain sits about eight metres from the ringfort bank, placing it firmly within the protected interior of what would once have been an enclosed farmstead or settlement.