It is the collapsed remains of a souterrain, an underground passage or chamber built during the early medieval period, typically beneath or beside a ringfort. These structures were constructed from stone and used variously for storage, shelter, or concealment, and thousands of them are known across Ireland, though many exist now only as subtle disturbances in the ground.

This particular souterrain sits within a ringfort, the circular earthen or stone enclosure that was the standard form of rural settlement in early medieval Ireland, roughly from the fifth to the twelfth century. The association between ringforts and souterrains is well established; the underground feature would have been accessible from within the enclosed domestic space above. At Rathredmond, what survives is a stone-lined hollow, the roof long since fallen, leaving only the rubble and the suggestion of what was once there. When it was surveyed for the 1994 archaeological survey of the Ballinrobe district, compiled by D. Lavelle, the feature was recorded as inaccessible, meaning even at that point the structure could not be entered or fully examined.

There is little a visitor could usefully observe beyond the earthwork itself. The collapsed nature of the souterrain and its position within the ringfort mean that the most the ground reveals is a depression in the turf, a quiet indication that something deliberate was once built here and has slowly given way.