I would do what I always do and say F*** the vatican, and then do whatever I wanted.
To officially leave the roman catholic church you must complete a form referred to as a "Defection by Formal Act". You can write to the archbishop requesting this and tell him under Vatican II, you wish to exercise your right to defect.... or you can print off the one I've linked.
You’ll need to provide some additional information like where you were baptised, your parents names, godparents names, and of course the date you were baptised (and if you don't have this, your date of birth will do)
So to summerise you should be able to print out, sign and send that defection form letter straight to the bishop of the diocese you were baptised in with at least
your name,
date of baptism,
parish church of baptism,
your date of birth,
your parents names,
and the local baptism records will be changed and you will be officially not catholic anymore (if you ever really were).
Reverend Dr Diarmuid Martin
Archbishop of Dublin and Primate of Ireland,
Archbishop's House,
Drumcondra,
Dublin 9.
Within a resonable amount of time you should receive a copy of your amended baptism certificate with defection written all over it.
