I've posted this before, and I'm sure I'll post it again but here goes:
First off, lucky you, you know not only what you want to do but where you want to do it. If you get it you are incredibly fortunate, but if you don't, don't worry. There's more than one way to skin a cat. If things don't go to plan at any stage be prepared to go at it another way. UCD, UL, UCC/CIT and WIT are all worth your consideration, it doesn't matter if you think that you might not like to go there, that you can't afford it or that you won't get the points, things change, you learn to love a place (and I say this as a Corkonian living in the despised Dublin), banks are our friends (to a certain extent!) and points change (they can go down too!)
As for the portfolio, go with what you know. If you're confident with the technical side of things go with it BUT I would say that that is probably of lesser importance than creativity - it's not uncommon to have NO technical stuff in a portfolio.
Do anything to start - that way you'll have plenty to choose from and even little scribbles can go into a sketchbook which they really like to see plenty of. They say that you're work has to be recent but some of mine came from my junior cert and noone remarked about it in any interview (you know the whole stamping everything gives it away!) If you are really clueless try a portfolio preparation course in PLC colleges or I did a good one in the Crawford in Cork at the weekend (I'm sure other colleges do them too). Not everything in you're portfolio has to relate to architecture (in fact none of mine did besides some photos - they really helped me to control what they asked me about and it only took a couple of hours with a camera around town).
As for the actual interview, I've been threatening for years to go down with study notes and blather on about how I don't know anything about the evolution of vernacular architecture in Brasil and watch the poor tortured souls who are waiting for interview freak out. In reality, you don't need to be a genius, if you have interests great! I got by on a few years of Grand Designs and a critique of the extension to the Crawford Art Gallery extension.
Having an opinion is important, know what you like and why, even if it's just your own school building. If you don't like something ask yourself why too. Keep abreast of any developements or relatively modern and high profile buildings in your area (don't worry if you haven't seen one thay ask you about, a no is fine too.)
Have a favourite building (not the Customs House in Dublin!)
DON'T UNDERESTIMATE THE IMPORTANCE OF THE SUITABILITY TEST for DIT, failing doesn't mean that you won't make a good architect but it's important to have it in the bag. The Suitability Test is usually in a few parts, a standard IQ patterns test, a personal statement style piece of writing, a small picture thing and i feel like there was something else but I don't remember!
Some things that I found useful were writing out a personal statement (I did mine for UCAS and it helped me to just get something focused down on paper quickly, I then wandered off in to a heap of rubbish about Munster Rugby so don't be too concerned about your literary quality!) and pattern related IQ tests ( again I did some in TY in school nothing too serious but the familiarity will help you to concentrate on the day).
Fianlly, consider applying to UCAS. I'm not sure when the application deadline is but have a look anyway, schools in the UK are just as good and better but many are far easier to get into. It takes a lot of pressure off knowing that worst come to worst you still have an offer from X, Y or Z. There are plenty of people around here who have or are attending uni in England and especially Scotland so they might be able to help with that end.
Good luck, stay calm and it will all turn out OK .... because that's just the way life is.