if you really want to do architecture (and certainly, you sound very enthusiastic about it: the first step!)... your choices are not as limited as they appear - have you considered or applied to the colleges in england, scotland... or 'queens' in the north?
i was in exactly the same situation as you 6 years ago: ie. i was not the typÃª to score 6 million points in the leaving cert. however, i did succeed in scraping into DIT, thanks to the aptitude test and interview.
the aptitiude test: hard to know what exactly they are looking for, and impossible to prepare for it.
if you get an interview: be enthusiastic, natural and (whether you have a portfolio or not)bring anything reflecting your creative side - drawings, photos, paintings... i even know some people who brought essays and poems they wrote!
you are not expected to have an incredible knowledge about architecture, but just be prepared to talk about any buildings, or cities that interest you - they want to see that you are spatially aware.
regarding entering other fields on the CAO(you mentioned civil engineering and the like?)... you must remember that you would have to complete the other course before (DIT) would consider taking you on(into 2nd year)ie. it's a very long process: but certainly not uncommon.
i'm not sure if any of this is of any use, i can only wish you the best of luck and to remain focused and positive on what you really want to do.