On registry offices, you can notify HSE of intention to marry at either Lombard St or Patrick Duns Hospital - and if you want to use registrar somewhere other than the actual registry office you can get a registrar from either (dont know if you can choose who you get or are you just assigned one) - but the only option to actually use a registry office for ceremony is Patrick Duns Hospital (in dublin anyway).
You should ring Patrick Duns and tell them you want to notify them of your intention to marry asap - they are very nice on phone (when you ring you leave message and they DO phone back that day) - chances are that the earliest opportunity for you to go and notify in person will be 3 months from now. Very earliest you can marry is 3 months after that appointment.
Its possible wait times are shorter outside of dublin (for notifying intention to marry). I dont know what the story is for registry offices outside of dublin, how many of them there are etc...
I used this website to get all this info:
http://www.groireland.ie/getting_married.htm
The first thing I did was phone Lombard St to tell them I wanted to notify them of intention to marry, when they heard I wanted to use the registry office for the ceremony they said to notify Patrick Duns of intention to marry instead as if I notified Lombard St, then they would have to liase with Patrick Duns etc...Easier to notify the place you want to use. So dont bother ringing Lombard St - go straight for Patrick Duns.
While that website has a lot of detail there was no substitute for the person on the phone who was able to clarify and simplify things for me within minutes.
When I told Patrick Duns I was ringing to notify of intention to marry they said earliest appointment to do so was end of November and I was to bring various bits, passport, long form of birth cert etc.. and that they would send me letter confirming appointment which would include list of necessary documents for appointment at end of November, they will organise wedding date at that appointment also.
They sent me the letter and they also phoned me yesterday just to confirm I was still going to the appointment (its next week - so I will have more info for you after it).