Cloyne would appear to be a reasonable guess.
I went on Roots Ireland www.rootsireland.ie and they have four John Geary baptismal records in Cloyne RC parish records between 1810 and 1820. You did not mention John's year of birth or the sibling names but you can look for their records.
Roots is a pay site to view the transcription of a church record.
I think based on the Roots info, Cloyne is the correct parish.