A souterrain, from the French for underground passage, is a type of dry-stone subterranean structure found across early medieval Ireland, most commonly associated with ringforts and thought to have served for storage, refuge, or both. What survives at Gorteen is, even by the standards of the type, unusually complex: three corbelled beehive chambers of varying size, connected by passages that turn, narrow, and descend at deliberate intervals, all constructed without mortar and with stones that, as of the late nineteenth century, appeared to be still in their original positions.

The fullest account of the structure comes from a description published by Falkiner in 1898, when the souterrain was situated on the property of Lord Congleton, in the parish of Moyliscar, roughly four miles from Mullingar and about 500 yards from the Roman Catholic church at Gainstown. The entrance, facing east and framed by large rough jambs beneath a single massive lintel, leads into a passage just under a metre square. After 5.2 metres the passage drops slightly, the roof rises to nearly 1.8 metres, and the route turns at a right angle into an ante-chamber where a flat stone shelf, spanning the full width of the passage, divides two onward routes. Below the shelf, a crawlway of only 0.75 metres square leads down to the first beehive chamber, 2.1 metres high and 2.7 metres across at the base, its apex sealed by a large circular capstone. Above the shelf, a second passage, partially obstructed by a broken lintel, heads west, passing a small offset chamber before curving gradually southward through 14 metres of total length to reach the largest chamber of all, 7.5 feet high and over 3 metres in diameter, its top closed by two overlapping flags. The whole system, passages and chambers alike, is built within the thickness of the ringfort's vallum, the raised earthen and stone bank that formed the enclosure wall, so that the underground structure is essentially hidden within the monument's own body.

The site lies on a low rise in undulating ground, and the ringfort itself remains visible. The eastern end of the souterrain passage is open, though the earthwork that once covered much of the structure has lost considerable height over the centuries. The deliberate engineering evident in 1898, the graded descents, the constricted crawlways, the branching layout, speaks to a builder's understanding of how to make a space both defensible and disorienting to anyone unfamiliar with it.