Souterrains, as these structures are known, are underground galleries cut into the soil or built from stone, typically associated with early medieval ringforts across Ireland. Their precise function is still debated, though storage, refuge, and concealment have all been proposed. What makes the one at Gortdonaghmore quietly compelling is the level of detail recorded before that closure, offering a remarkably clear picture of a space most people will never be able to enter.

The description comes from J. Coleman, writing in 1947, who accessed the site through a deliberate opening cut at the northwest end of the first chamber. What he found was a sub-rectangular, earth-cut room oriented roughly north to south, measuring around eight and a half feet long, three feet wide, and not quite four feet high; a space you would crawl through rather than walk. A creepway at its southwest corner connected to a second chamber, this one oriented east to west and slightly wider, at roughly six and a half feet long by three and a half feet wide. Both chambers were already partially filled with earth and stones at the time of Coleman's visit. Running eastward from the north end of the first chamber was a stone-built passage, over twenty-two feet long but only around eighteen inches high, far too low to be used as a thoroughfare in any conventional sense. At the eastern end of this passage, Coleman identified an oval-shaped depression, which he interpreted as the original entrance to the whole complex. The surface above the passage is still marked by a visible depression, with four stone slabs exposed to view. Coleman also noticed, about seven feet to the south, a hollow roughly five feet across with traces of stone walling on its western side, which he cautiously described as a possible further chamber or the remains of a hut site.

The entrance to the souterrain is now closed, and the interior is no longer accessible. The ringfort within which it sits, however, remains part of the landscape, and the low depression running across the ground above the filled passage is still faintly visible as a surface feature for those who know to look for it.