It's the same process for ordering birth certs - you can use the same order form.
Fill in your details (name & address/credit card etc) on the order form, cross out the other details and write 'See Attached' across that section of the form.
On a separate page note the references for various certs you require. Also mention you require 'research certs' or 'photocopies' . For each record, the GRO need the following details :
name on the cert (either will do for a marriage),
type of record - i.e. Birth, Marriage or Death
year, or year and quarter if given
registration district
volume number
page number
As long as these references are given, and correct, then no searching required and each cert will cost you €4. Research certs are photocopies of the original GRO register.
see :
Details included on a Birth CertShane