The site is listed as a souterrain, an underground stone-lined passage of the kind built throughout early medieval Ireland, typically associated with a nearby settlement and used for storage or refuge. What makes the Carrigthomas example quietly peculiar is that it has vanished twice over: first underground, as it was always intended to be, and then in a more final sense, when the passage was infilled with stones and left with no visible surface trace whatsoever.

The record of this place depends almost entirely on a brief note made by Hartnett in 1939, which described the remains of a lisheen, a small enclosure or ringfort, with an associated subterranean passage. Hartnett's account drew on local descriptions rather than direct observation, since by that point the passage had already been filled in. Those descriptions suggested a construction of the stone-lined, flag-roofed type, which is the most common form of souterrain found in Munster, where upright stone slabs form the walls and large flat stones are laid across the top to create a covered corridor. The lisheen with which it was associated, a term used in Irish for a small fairy mound or minor enclosure, points to a settlement of some kind once standing nearby in this low-lying, waterlogged ground.