The discovery was made in 1793 by labourers digging a fosse, a defensive or boundary ditch, around a new church at Kilbixy being built for Lord Sunderland. When they broke into what lay beneath, they found not just a passage but an entire complex of interconnected cells, some oval, some circular, ranging from roughly six to eighteen feet in diameter, each connected to the next by small arched openings. More striking still was the presence of horizontal channels built into the masonry, thought at the time to serve as air vents. The original account, published in 1793, noted that the underground complex covered over a quarter of an acre, with new chambers still being uncovered as the report went to press.

A souterrain is a deliberately constructed underground structure, typically associated with early medieval Ireland, built from dry stone or cut into bedrock and used for storage, refuge, or both. The Kilbixy example, if that is what it is, would fit the general form, though its scale as described is remarkably large. The 1793 account captures something of the bewilderment felt at the time: the writers confess themselves at a loss to explain the purpose of the air funnels or the cells themselves. The fosse dug for Lord Sunderland's church is thought to have cut across the souterrain somewhere to the north-east of the graveyard, which gives a rough indication of where the complex might lie, but its precise location has never been established since that initial, excited excavation.