Fair dues to you for copping on! Better now than this time next year anyway!
There are some students who get A's for five years and don't end up with 520 points, so just so you know you have a mammoth task ahead of you, but if you can keep this dedication going all through the summer, then there is no reason why you can't achieve it (or close enough to get into the field you want anyway).
First thing you need to do is make a plan. Write down your list of subjects and the topics/ chapters you have already covered. Any that you were particularity weak in, you need to revise. Don't just read over them, do a few questions too. Find a style that works for you- note-taking, reading aloud, flash cards etc.
Even if your teachers haven't said so, it would be no harm to get a set of exam papers (or just look them up on examinations.ie) and see what style of questions you need to know for that topic. Controversial notion, but if it's points your after (and not just knowledge for the sake of knowledge), then you have to play the system as it were- learn answers off by heart, pre-plan essays, that kind of thing.
Once you get back to school, develop a different routine. Mention to your year head that you want to be more exam focused, and you would appreciate any help s/he, or your teachers, can offer. Have all your books and copies neat and with you. Pay attention in class, ask questions if you're lost, start talking to teachers after class if you want extra notes/want them to correct something for you. Do you do after-school study? Use this time to get homework done, come home, have dinner, then another 1.30/ 2hr studying/extra questions time. Once you're in the right frame of mind and in a pattern, it'll get easier. Get some of the Revise Wise/ Less Stress books too.
I'm assuming, since you're failing at the minute, that you're not naturally academic, so maybe the people around you won't be used to seeing you work like this. This is going to sound so bad, but the main trap I see students fall into is listening to soft teachers/parents say ''Oh, you're getting too stressed, you'll burn out, take a break for yourself'', when pupils are putting in a few hours here and there, and getting a little flustered. Look, it's not supposed to be easy, get that into your head, it is stressful and difficult. Obviously if your mental health is suffering that's one thing, but always be pushing yourself to work harder. There are students studying for the two years from 6am-10pm, and they will be sitting the exact same paper as you, all competing for the same places.
You do not want to get your results that August and see 490 or something like that.
Anyway, if you need any help/notes PM me, I'm happy to help.