Beneath the garden of an 18th-century deanery on Cathedral Square in Waterford city, a medieval undercroft sits largely out of sight and out of mind, yet it is one of the more complete surviving examples of 13th-century ecclesiastical architecture in the south of Ireland. An undercroft is a vaulted ground-level or semi-subterranean chamber, typically used for storage or service functions beneath a more important building above. Here, the upper structure has vanished entirely, leaving only the lower storey, sealed for centuries under accumulating earth, then forgotten, then rediscovered.

The earlier of the two interconnected chambers dates to the 13th century. It measures just over twenty metres north to south and nearly six metres wide, with a barrel vault supported along its centreline by an arcade of six arches springing from five square pillars of Dundry stone, a distinctive pale limestone quarried near Bristol that was widely imported into medieval Irish towns as a prestige building material. The original entrance was a pointed Dundry stone doorway with a newel staircase at the south-west corner. Four window embrasures open in the west wall, with a fifth in the east. Attached to the north, and probably added in the 15th century, is a second vaulted chamber of similar width but somewhat shorter, its south-east section damaged when the 18th-century deanery was built above and its staircase inserted through the vault. Documentary references that likely relate to this later structure appear as early as 1468. The earlier undercroft was blocked off and abandoned sometime in the 17th century, lying sealed until the 1840s, when it was reopened and roughly half a metre of floor deposit removed. Excavations carried out in 1986, independently by C. O'Rahilly outside the structure and M. O'Brien within it, revealed the original stone floor of the 13th-century chamber and evidence of a 15th- to 16th-century yard on the east side, associated with a later inserted doorway.