Popularly known as the Synod Hall, this rectangular choir measuring roughly 21 metres in length was never quite what it was meant to be. It appears to have been conceived as the first phase of an ambitious medieval rebuilding of the entire cathedral complex, a scheme that, for reasons now lost, was simply abandoned partway through.

Construction of the Synod Hall dates to the early decades of the 14th century, carried out under Archbishop William de Bermingham. It was built on the same alignment as the north wall of the older Romanesque cathedral it adjoined, the two structures sharing an axis that still shapes the building today. The choir's side-walls are pierced by tall pointed windows with double and triple tracery lights, and the east gable holds a particularly fine five-light traceried window. Externally, weathered buttresses flank the walls, and a crenellated parapet, reconstructed in the 19th century, runs along the top of an arcaded corbel course. For a time, a slender tower described as friary-like, probably dating to the 15th century, rose above the junction of the Synod Hall and the old Romanesque chancel. It was removed during the Victorian works that produced the present cathedral, leaving only a stump. The Romanesque chancel itself was repurposed sometime in the 18th or early 19th century as a west porch, layering yet another phase of reuse onto an already complicated site. Excavations along the north wall in 1986 uncovered wall foundations and burials, among them two individuals interred with scallop shells, the traditional badge of pilgrims who had made the long journey to the shrine of Santiago de Compostela in north-western Spain, a small detail that places this corner of Galway on a medieval European religious network stretching far beyond the town.