Imposing Ballintubber Abbey is the only church in Ireland founded by an Irish king that remains in use. It's reputed to have been established in 1216 next to the site of an earlier church founded by St Patrick after he came down from Croagh Patrick. Its history is tumultuous: the abbey was burned by Normans, seized by James I and suppressed by Henry VIII. It's signposted off the N84, 13km south of Castlebar.
The nave roof was burned down by Cromwell's soldiers in 1653 and not fully restored until 1966, but the apse was the site of services for hundreds of years, in all manner of weather; indeed, the abbey has provided Mass continuously to the present day for 800 years, the only church in Ireland to have done so.
The ruined cloister is also a sublime sight, while Grace O'Malley's son, Tiobóid na Long (the 1st Viscount of Mayo), is buried in the vault. Guided tours and a 25-minute video help tell the abbey's story. Tóchar Phádraig (Patrick's Causeway), the ancient 35km pilgrim's route to Croagh Patrick, begins at the end of the churchyard.