Fionn:
I searched the subscription site Roots Ireland and the only Magan/McDermott marriage was on April 17 1825 in Boyle RC parish between Patrick McDermott and Mary Magan. The records for people in Cloonfindlough civil parish would be part of Strokestown RC parish and their records start in 1830.
I also searched for McDermott baptismal records with a mother with the maiden name of Magan and got no record.
Roger McDonnell