I presume you mean the HDip i.e. the qualification for secondary teaching?
If so, getting a place is no pushover but not impossible either. Obviously you're too late for this year but I'd make a few phone calls to the colleges to check when its advertised. AFAIK, all colleges advertise seperately but I'm not 100% sure on that. Its offered in UCD, Trinity, NUI Maynooth and Galway and possibly UL and UCC.
Having a bit of teaching experience is an advantage, but not essential. If you could get some sub-work it'd stand to you. Do you speak Irish by any chance? NUIG runs an Irish language version which is the course I did myself and I would highly recommend it! Even though I ended up not going into teaching, I find the course has stood to me ever since and as a stand alone exercise, was worth doing for its own sake. Best of luck with it.