Its construction was undertaken by Dónall Mór O'Brien in 1185. It was built over the site of a monastery founded in the 6C by Saint Molua and dedicated to his successor. Work on the cathedral began in 1225, in a combination of Romanesque and Gothic styles that can still be seen today. It is worth noting the Romanesque doorway and a 12C High Cross sculpted around the year 1000 requesting a prayer for Thorgrim, which is engraved in both Ogham and Viking Runic characters.