A souterrain, to give it its proper name, is a type of man-made subterranean structure found across early medieval Ireland, typically constructed from drystone walling and roofed with large capstones. They were dug and built, most likely between the seventh and twelfth centuries, to serve as places of refuge, cool storage, or both, and they are often found in close association with ringforts and other settlement sites of the period. The one at Cloontubbrid is recorded as a monument, which tells us it was identified and noted at some point, though the details of its form, dimensions, and condition remain largely uncharted in publicly available sources.

Mayo has no shortage of such structures. The county's landscape, shaped by centuries of small-scale farming and periodic upheaval, preserves a surprising number of these underground features, many of them still partially intact beneath fields that have been worked continuously since the early modern period. A souterrain might announce itself through a slight depression in the ground, a scatter of displaced capstones, or simply a hollow sound underfoot. The townland name Cloontubbrid derives from the Irish, likely incorporating the element cluain, meaning a meadow or pastoral area, which suggests this was productive agricultural ground in earlier centuries, the kind of landscape where a defended homestead and its associated underground chamber would have made good practical sense.