Tucked off Conduit Lane on its eastern side, a medieval barrel-vaulted chamber survives in Waterford City in a state that invites more questions than it answers. Barrel vaulting, a technique in which a continuous curved ceiling of stone spans a rectangular space, was used in medieval Ireland for everything from undercrofts beneath merchant houses to sections of religious buildings and defensive structures. This particular example measures twelve metres long, four and a quarter metres wide, and two and a half metres high, making it a substantial space by any standard, yet it sits quietly off a lane without the fanfare that usually accompanies Waterford's better-known medieval fabric.

The chamber is entered through an opening in the north wall and lit by a window set into the east wall. Its association with Conduit Lane is itself suggestive. Medieval Waterford had a sophisticated water supply system, and lane names in the old city often preserve traces of the infrastructure or land use that once defined a particular area. Whether this vaulted space served as a storage cellar beneath a larger house, a warehouse, or something else entirely is not recorded. What the 1989 Urban Archaeological Survey of Waterford City and County noted, when it documented the structure, was the physical fabric: the dimensions, the entry point, the window. The interpretation of how it was actually used in daily medieval life remains open.