the order form to download is on the GRO website - see :http://www.groireland.ie/apply_for_a_cert.htm
print that out and fill in your details - name, address, credit card details etc. Ignore the other sections of the form and attach a separate page with the references from the index - i.e. the name, event type, year/quarter, district, volume & page. Mention your require a 'research cert.' These cost just €4, and are a photocopy of the GRO register, and post the pages to the GRO head office in Roscommon.
A marriage cert should include a few detail that should help the search - e.g. current residences of the bride and groom, father's names, occupations, names of witnesses etc.. plus date and location of the church, or registry office.
Which of the children are you descended from ?
It's a good idea to confirm the links - e.g. with a birth cert and/or marriage cert for your link in the next generation - i.e. your grandparent