A souterrain is an artificial underground passage or chamber, typically built during the early medieval period, constructed from stone and roofed with large lintels. They appear across Ireland in their hundreds, associated with ringforts and early settlement sites, and scholars continue to debate their precise function. Refuge, food storage, ventilation shaft for a dwelling above, perhaps all three depending on the season or the century. The one at Lahardaun is recorded as a monument, and that simple fact alone suggests something survives, or once survived, beneath the surface.

The broader Lahardaun area sits at the foot of the Ox Mountains, a landscape that was well settled in early medieval times and carries layer upon layer of prehistoric and historic remains. Souterrains in this part of Connacht were often constructed alongside ringforts, the circular enclosed farmsteads that were the basic unit of rural life from roughly the fifth to the twelfth centuries. Stone-built examples in the west of Ireland tend to reflect the local geology, using whatever flags and boulders lay readily to hand. Without more detailed records currently available for this specific site, the precise form of the Lahardaun souterrain, its dimensions, the number of its chambers, and the condition of its stonework, remains uncertain.