Gracing the River Boyle is the finely preserved (and reputedly haunted) Boyle Abbey. Founded in 1161 by monks from Mellifont in County Louth, the abbey captures the transition from Romanesque to Gothic style, best seen in the nave, where a set of arches in each style face each other. Guided 40-minute tours of the abbey are sometimes available.

Unusual for a Cistercian building, figures and carved animals decorate the capitals to the west as well as, more bafflingly, the pagan sheila-na-gig fertility symbol. After the Dissolution of the Monasteries, the abbey was occupied by the military and became Boyle Castle; the stone chimney on the southern side of the abbey, which was once the refectory, dates from that period.