A souterrain is an underground stone-built passage or chamber, common throughout early medieval Ireland and typically associated with nearby settlement, used variously for storage, refuge, or ventilation. The Brackcloon example is unusual partly for its scale and layout, and partly because one archaeologist suspects it was never entirely a souterrain at all.

Discovered in 1931, the structure was described in detail by S. P. Ó Ríordáin in 1934. He recorded four rectangular chambers, all cut directly into the bedrock rather than built up with dry-stone walling, which already marks the site as distinctive. Chamber 1 sits at the hub of the arrangement, with openings leading off to the west, south, and east into chambers 2, 3, and 4 respectively. Entry to the whole complex was made from the east side of chamber 4, down four rock-cut steps through an unroofed annexe. A drainage channel, roughly 24 centimetres deep and 20 centimetres wide, runs diagonally across chamber 4 in a south-west to north-east direction, then continues northward through a rock-cut trench nearly twelve metres long. That trench reaches a maximum depth of just over one and a half metres. Writing in 1977, McCarthy proposed that chamber 1 was not an original underground space at all, but a construction shaft, sunk to give workers access while the rest of the complex was being carved out. If that reading is correct, what looks like a central room was actually a means to an end, a working hole that later became incorporated into the structure it helped to build. Today, three surface hollows and a depression roughly seven metres long mark where the chambers lie beneath the hillock, the underground geometry made faintly legible from above.