The main teacher-training colleges (sorry, Colleges of Education!) have their B.Ed and Grad. Dip intakes set by the DES each year, presumably on the basis of projected job openings three years hence. Those figures have been slightly increased in each of the last few years, so I'd imagine prospects are fairly good generally - but a lot would depend on what geographical area you were hoping to work in.
Yes, primary (and secondary) schools regularly recruit unqualified subs. But I don't think they're allowed employ them fulltime any more, unless in exceptional circumstances. You could have a look at the relevant [broken link removed] (see point 6., p.4) and other info on the INTO website.