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I think there were two John Harrington/Mary Mahony couples. Roscarbery is a good distance from Schull East so I don't believe the family moved from Roscarbery to Schull. When you match up the children baptized in Schull separately from Roscarbery, you see there are seven in Schull with the latest in 1839 and seven in Roscarbery with the latest in 1841. I would stick with the Schull records which means not having a marriage record. You could scan the marriage records on the parish register.
Let me know what questions you have.