St. Cronan had originally founded a monastery at Sean Ross, a mile east of the town, but this proved too difficult to find as it was away from the main roadway. He instead moved back closer to the main road, the Slí Dála, and founded a new monastery there.
Originally the monastery was built of timber, but the ruins of the stone church were built in roughly 1100 AD. Built in the Romanesque style, all that remains of the abbey is the west wall. For a short time Roscrea was an independent Diocese but this was short lived and it was eventually joined into the Diocese of Killaloe shortly after 1161 AD. It was here that the famous Book of Dimma was written in the 8th century.