Going back to that original question - should OP go with an ATI apprenticeship now and later switch to full accountant training or should OP do a related degree and then join an accounting firm while preparing for his professional final exams.
My first thought was that this choice of ATI qualification seems odd: if you are good enough to be a candidate for UCD commerce/accounting/economics you should be looking at an
ACA articleship or apprenticeship as they call it today. As you see on the CAI page, your relation to the ATI programme need only last 1 year, the second year is optional - perhaps something those with no previous knowledge in accounting or business subject might want to avail of. I doubt if this would apply to you as you have studied the stuff at school.
So as I see it, your choice would be between the Accounting/Finance degree or the ACA apprenticeship.
Misemoi tells us of the Central bank scholarships which start at €27,000 (that's not to be sneezed at) and involve a Griffith College degree. A little bit of (economic) independence is needed to get the habit of responsibility - something most students don't seem to have or if they do can't express it in the sea of loud vulgarity known as student life. An 18 year old being a net provider to his home rather than a net expense will give you a status of sorts at home. I see no explicit mention of the Central Bank supporting your studies towards professional qualifications like ACA - but you can check up on that.
But there are plenty of accounting firms offering a similar sort of proposition, though maybe not as well paid as the Central Bank's one.
Previous posters have advised to get a degree and then train with a big 4 accounting firm.
I'm not an accountant so all I can say is what
Michael Deeny said once about choosing a firm to train with: in a small audit firm you get to see all sorts of businesses and how they make their profits, in a large firm you can spend all your time working on the accounts of the same 3-4 huge companies. Granted Deeny never intended to be an accountant for long, he just wanted to get into business. If you want a professional career in audit and feel at ease with people from the corporate sphere then you may make a different decision.
So it's college life, Erasmus, academia, etc vs the pay, adult working environment and well-supervised progress of an apprenticeship in an audit firm. Only you or your parents (possibly teachers too) who have been looking at you and your motivation through your life can judge what is right for you.
One bit of advice I will give you regardless of which path you choose: always do regular exercise 6 days a week. This can be casual sport or just a workout on the floor at home though it's naturally more a total experience if it's with others in a gym or sportsfield. But get it done as it make you feel better and think clearer.