The opening, roughly a metre wide and less than half a metre deep at its threshold, leads into a souterrain, one of the underground stone-lined passages or chambers that were constructed throughout early medieval Ireland, most likely for storage, refuge, or both. What makes the site quietly odd is less what it contains and more what obscures it: the entrance has not simply aged into ruin, it has been swallowed, its outline surviving in the records but not easily in the landscape.

The souterrain sits within a rath, the type of circular earthen enclosure that served as a farmstead during the early medieval period in Ireland, roughly the fifth to twelfth centuries. Raths are among the most common archaeological features in the Irish countryside, though most people pass them without recognition, reading them as odd circular rises in a field. Souterrains are frequently found within raths, dug beneath the enclosed area and accessible from inside, which is consistent with what survives at Letterdunane. The lintelled entrance here, stone slabs laid horizontally across the opening to bear the weight above, represents a straightforward but durable form of construction that has kept the passage mouth identifiable even as the surrounding growth has done its best to close it off entirely.