St Mary's Cathedral is Tuam's most important stop. The present Church of Ireland cathedral is mostly 19th-century, but it preserves major older pieces, including a Romanesque chancel arch from the first cathedral and the 14th-century Synod Hall.
The reason not to miss it is the High Cross of Tuam. The cross was assembled from medieval fragments, displayed in Dublin, returned to the town, and eventually moved inside St Mary's to protect it from weather and traffic. It gives the whole town's ecclesiastical story a physical centre.