You have to give the 3 months notice of marriage required by law - (this goes for all marriages, not just registry office) - but you do not necessarily know the exact date at this stage, just that the wedding will be a min of 3 months after you submit the paperwork. You then have to meet with the Registrar in person when the 3 months is up & give birth certs, proof of address etc. Date of the wedding is usually set at this time and can be anytime afterwards, subject to availability of registrar etc.
Registry Office will supply you with forms etc. you need and tell you when and where to send them.
One thing to bear in mind is that Registry Office weddings have become a lot more popular in recent years due to the dramatic rise in the number of people getting married abroad and this is causing some delays with waiting lists of months in some areas, so advice is do the paperwork early. Usually people getting married abroad have a civil ceremony a few days before they go abroad (just couple and 2 witnesses & guests at the wedding abroad are not even aware of this) and the foreign ceremony is just a religious one. The reasons being that (i) its better to have your legal paperwork in english in Ireland, (ii) in many countries e.g. France, a religious ceremony has no legal status, therefore they need to do a seperate civil ceremony anyway and (iii) in some countries (some non-EU), weddings do not necessarily conform with the legal requirements to recognised as married in Ireland.