The GRO research room in Dublin has recently moved to Werburgh St. in South Dublin - near to Dublin Castle. You can access the Index books (not the registers themselves) in the research room and order certs based on the references for €4. Up to 5 certs may be ordered per person, per day.
You dont need to go to the research room if you already have the index references, as you can use these to order research certs by post from the GRO head Office in Roscommon. These show the same details, and are the same cost as those available in the research room.
You can download the order form from : http://www.groireland.ie/apply_for_a_cert.htm