This kind of structure is known as a souterrain, an artificially constructed underground space typically associated with early medieval settlement in Ireland, and generally thought to have served as a place of refuge, food storage, or both. What makes the Froe example particularly interesting is the variety of building techniques used within a single structure: the first chamber is stone-built, chambers four and five are cut directly from the earth, and the two in between combine both approaches, suggesting either phased construction or simply the pragmatic use of whatever method suited the local ground conditions at each point.

The souterrain came to light in 1981, recorded by Cleary, and the five chambers vary considerably in size. The third chamber is the longest at 5.7 metres, while the first is the most confined, barely 80 centimetres high and just over a metre wide, a space that would have been uncomfortable to move through even by the standards of people considerably smaller than the modern average. Chamber five, the furthest from the probable entrance on the north side of chamber one, is also the tallest at 1.5 metres, and contains what is described as a built-up construction shaft, a feature that would have been used to remove spoil during the digging and building process and then sealed or modified once the work was complete. That this shaft survives in recognisable form gives some sense of how well the structure has been preserved underground.