It is the kind of detail that stops you mid-stride, and it sits alongside a quatrefoil window, a form more associated with decorated Gothic than with the severe plainness you might expect from a building with Cistercian origins. The structure itself has passed through so many hands and so many uses that reading its fabric becomes something close to an archaeological puzzle.

The site began as a small Cistercian chapel in the thirteenth century, dedicated to Saints Peter and Paul, and may have served as the parish church for the medieval borough of Portumna. At some point it fell out of use, and before 1414 it was granted to the Dominican friars by the O'Maddens, a prominent Connacht family. The Dominicans rebuilt extensively, and most of what survives dates from the fifteenth century: a long nave and chancel separated by a tower that has since disappeared, a south transept, a refectory in the north range, and a cloister arcade, the west and north sides of which have been reconstructed. Two slender opposing windows with pointed arches in the chancel walls are older, survivals from the original Cistercian building. The fifteenth-century work is more elaborate, including a four-light traceried window in the east gable of the chancel and a three-light traceried window in the south wall of the transept. A piscina, a small stone basin set into the wall and used for disposing of water blessed during Mass, remains in the south wall of the chancel. After all of this, the building was converted for Protestant worship in the eighteenth century, adding one more layer to an already complicated history.

The friary lies about two hundred metres southeast of Portumna Castle, within the demesne grounds to the south of the town. Inside the chancel, a series of medieval graveslabs are still visible, worn but legible enough to reward a close look. The sacristy doorway with its carved head is easily missed if you move through the site quickly, so it is worth circling the building before settling on the larger windows.