There's quite a few John Joseph Lynch births in 1926 (see
link) - assuming he was registered with the middle name and the year of birth is accurate .. but the best match seem to be this one :
Name: John Joseph Lynch
Registration district: Tipperary
Event type: Birth
Quarter and year: Jul-Sep 1926
Volume : 3 / Page : 426
The registration district of Tipperary covers the town of the same name, plus quite a number of towns and townlands in the area.
Assuming that we have the correct father's name, Christopher was the eldest in the family, and the couple married in the same area not long before his birth (i.e. 1919-1923) then there's a
possible marriage match on the BMD index :
Name: James Lynch
registration district: Tipperary
event type: Marriage
Quarter and year: Jan-Mar 1920
Volume : 3 / Page: 447
Name: Annie Kelly
[same references as James]
It's a long shot... but might be worth trying certs for these while you are sorting out the marriage cert.
Ordering Certs from GRO Roscommon Details included on a Marriage CertShane