A lot of this has been covered in similar threads. Check them out. Also check out the wannabes forums on pprune.org.
The best way is to have lots of money to pay for the training.
Try the Air Corps is you like but it's tough to get in. Most people end up in one form of flight school or other.
Flight school costs from 50,000 Euro if you do it part time up to 100,000 Euro full time. By the time you leave school, you can add a few grand to those totals.
You don't need any points in the leaving cert. In fact I would say that you don't even really need a leaving cert. In fact most airlines could care less about the leaving cert. But do it anyway. You'll need it to go to college so you can get a well paid career so you can get the money to learn to fly.
No there are no grants, if only.
Some flight schools will help you get a loan but that's it.
Oh and by the way, Aer Arann and Ryanair expect you to pay another 20 to 25k for their training if they accept you.
It's all good news, isn't it?