I agree with BM. The portfolio is more of a talking point during the interview. It serves the purpose of giving them something to talk to you about. It also demonstrates that you can think in three dimensions and are spatially aware.
If you do not have a portfolio, you can talk to the interviewers about places you ahev been to and how they appeared to you, exhibitions that you have visited. You could visit the RIAI and see the various exhibitions that they run there on architecture. The new national gallery extension would also be weel worth a visit. If you spent an hour in there walking around and were then able to talk to an interviewer about how the spaces made you feel and whether you liked them or not, it would make a good interview conversation.
You should also check your local library and ask for books on architecture, either ancient or modern, although modern would be abetter bet. Read them, see how you feel about them. you could make a list of them or take copies of the relevent material and bring it with you to discuss. The ability to discuss is the most important part of the interview.
Remember most people who come with a portfolioto the interview are bringing the work that thy have done in an art class at secondary school. I don't know about every art class obviously, but mine was pretty standard, still lifes, life drawing, craft work, images of 3d work such as pottery, etc. none of these showed the interviewer that i had the potential to be an architect.
Talking about danish architecure, european cathedral building, my favourite place in dublin, industrial design of taps and other stuff that came up during the interview probably gave them a much better idea of potential.
PS i remember that day of the interview, and while most of us were dressed in suits, ties and all that, lugging A1 portfolio cases along the ground, others turned up in casual dress, no portfolio either and i met many of them again on the first day of college, including BM if i am not mistaken