In the graveyard immediately east of St Brigid's Cathedral in Kildare town, eleven steps lead down from the surface into a small stone chamber that most visitors walking overhead would never suspect was there. The room is modest in every dimension, measuring just over four metres north to south and barely three metres wide, yet the way it sits below the present ground level conceals something quietly telling about how the landscape around it has changed over the centuries.

The chamber is barrel-vaulted, meaning its ceiling forms a continuous curved arch running the length of the room, and it was constructed using wicker-work centring, a technique in which a temporary framework of woven rods was used to support the stonework while the vault was built and the mortar set. The entrance is a rectangular opening, or ope, in the west wall, and the flagged floor inside appears to have been raised at some point, further lowering the effective headroom. The most revealing detail is a flat-lintelled window with an internal splay set into the south wall. A splayed window, wider on the inside than the outside, is a common means of drawing more light into a thick-walled building, and its presence here tells us that when this structure was first built, it stood above ground. The earth has risen around it over time, or the building has gradually been absorbed into the accumulated layers of the graveyard, until what was once a surface-level room became an underground one. Bradley et al., writing in 1986, recorded these dimensions and details, and the structure remains one of the less examined features of an ecclesiastical site otherwise well known for its associations with St Brigid and the long history of the cathedral beside it.