Hello Terry!
The records for Murroe-Boher/Abington start in 1814 http://registers.nli.ie/parishes/0237 Probably too late for your John. I did find two records in Limerick city in 1803 and 1804 which are below. John Mahoney/Mahony was a very common name and unless you know his parents names, it would be difficult to confirm any record you locate.
Roger
First name(s)JohnLast nameMahonyBirth year-Baptism year1804Baptism date13 Feb 1804ParishSt. Michael's, Limerick cityAlternative parish namesSt. Michaels, LimerickDioceseLimerickCountyLimerickCountryIrelandFather's first name(s)JohnFather's last nameMahonyMother's first name(s)MaryMother's last nameMahonyRepositoryNational Library of IrelandNational Library of Ireland linkhttp://registers.nli.ie//registers/vtls000635021#page/1/mode/1upRegister
First name(s)JohnLast nameMahoneyBirth year-Baptism year1803Baptism date20 Mar 1803ParishSt. Mary's, Limerick cityAlternative parish namesSt. Mary's, LimerickDioceseLimerickCountyLimerickCountryIrelandFather's first name(s)JohnFather's last nameMahoneyMother's first name(s)MaryMother's last nameDoranRepositoryNational Library of IrelandNational Library of Ireland linkhttp://registers.nli.ie//registers/vtls000635016#page/1/mode/1upRegisterBaptism