Your search is complicated by the fact that we don't know for sure if the father of John Carew was living in 1850 and leasing land when the Griffiths survey was conducted. There are 23 Carew records in the Griffiths for the entire county but it appears there are many duplicates and the number of unique individuals was likely around 10-12 and about half are in Ringagonagh civil parish.
Looking at the subscription site Roots Ireland there are nine John Carew baptismal records in Co. Waterford from 1821-1827 but none in 1826.
The RC parish for Ringagonagh civil parish would be Ring and of the nine records I mentioned, only one was in Ring RC parish.
John Carew baptized May 5 1822 father James Carew and mother Joanna Savage. The family lived in Helvick townland and when you go back to the Griffiths, there is a James Carew in Helvick (actually two records indicating he had two separate parcels of land).
Is it possible that John Carew adjusted his age in order to be 21 rather than 25?
Let me know what you think about this possibility.