Newtown Abbey, properly the Cathedral of Saints Peter and Paul, is the main survivor of an Augustinian abbey founded in 1206 about a kilometre downstream from Trim. It fell derelict after the Dissolution in 1537. The smaller church just to the east holds the tomb of Sir Lucas Dillon and his first wife, whose effigies lie separated by a sword; locals call them "the jealous man and woman".
Across the river stand the ruins of St John's Priory, founded in 1202 by the Crutched Friars, who ran hospices for the sick. The narrow St Peter's Bridge between the two is controlled by traffic lights. The walk out along the river from the castle is the best way to get here, and both sites are free and open at all hours.