Hello there
First,don't wait til you're 22 or 23.You'll be a different person then and might not have the interest in flying/having a military career that you think you have now.Apply after you have seen your LC results, so that if you don't get in the first time, you'll have room,age-wise, to try again.Go and talk to current Air Corps pilots and get a real insight into what an AC pilot actually does.It's a lot different than you might think.Flying is only part of the job, not the full job like a civil pilot.Do as much research as you can on an Air Corp pilot's routine before you ever reach the interview.
If you haven't ever flown, go and get an intro flight to see if you really like it. You might actually hate it,you never know.
Also, it reads in your post as if you want to be an academic or hold down a business-related job first and be a pilot second. Becoming an AC pilot is tough enough, unless you are determined enough and prepared to spend 6 months in the Curragh before you even see an aircraft,let alone sit in one. There's no shortage of study and plenty of pressure to succeed.You absolutely have to want the job, really,really want it and be prepared to work very hard.As an aside, if you really want to be a pilot, consider borrowing and funding your civil licenses.
Australia can wait.It won't go away and it's already chocka with Irish.Service in the RDF proves only that you can respond to discipline, but it can't hurt.The interview board will want to see an intelligent, maturing individual with a credible range of interests and a record of achieving results.Again, get some interview technique practise beforehand.
The Defence Forces are very active in promoting their careers, at school and college level.Your careers guidance teacher should be able to sort you out there.If not, contact the Air Corps PR and they'll direct you.
Good luck.
GttC