Welcome to the board John.
Whether or not previous flying experience gives you an advantage in the competition is a much debated issue. Some say it does, but some say that the Air Corps prefers to train it's pilots from scratch (i.e. their way of flying), its very hard to know but I think it can't hurt to show that you have got some hours. With regard to your leaving cert, as long as you are doing a European language (french, german, spanish) then you should be okay.
Hope this has been of some help to you,
Matt_k