A souterrain is an underground stone-built passage or chamber, typically constructed during the early medieval period and associated with ecclesiastical or domestic settlements; they served purposes that remain debated, including storage, refuge, and drainage. This one was uncovered during archaeological test-excavations carried out by Cóilín Ó Drisceoil in the area known as the Deanery Orchard, a former walled garden that had been converted into a public amenity space.

The chamber itself is rectangular, measuring 1.65 metres east to west and at least 1.5 metres north to south, and includes a niche set into the west wall. It was built using clay-mortar bonded stone and cut down into natural boulder clay, slicing through the levelled bank of the enclosure that once defined the early medieval ecclesiastical settlement of Cill Chainnigh, the Irish name for Kilkenny. In that period, the Deanery Orchard formed part of the southern precinct of this settlement, a community of which St Canice's round tower is now the only upstanding structure. Excavations across the site have gradually revealed the scale of what lies beneath: a bank and fosse, the earthwork boundary of the ecclesiastical enclosure; a 9th-century stone-lined cess-pit; pits with evidence of bone and antler working dating to the 10th through 12th centuries; and burials beneath the round tower radiocarbon-dated to the 10th and 11th to mid-12th centuries. The souterrain's chamber had been filled with waste material, including fragments of 15th-century line-impressed tiles, before the late 17th-century northern boundary wall of the Deanery Orchard was built directly over it. The structure does not end there, however. It continues northward beneath Church Lane and almost certainly extends further into the cathedral graveyard, meaning a significant portion of it remains unexcavated and inaccessible, preserved in situ beneath the ordinary surface of the city.