This is one of the oldest parts of the building. Before becoming the sacristy of the present Cathedral it was the Chapel of St Andrew built in 1825 by the first resident priest, Father William Foley. It was dedicated to St Andrew as this had been the name of the original medieval Priory in the vicinity of which the Cathedral now stands.

The original altar still stands in the bay which is surmounted on the outside by the stone statue of the Sacred Heart visible from Barrack Road.

The choir stalls are from the Cathedral before the sanctuary was reordered in 1998. The sacristy is used for Mass on weekdays and as well as for meeting of the Cathedral Chapter