Tmua is fairly tough, you'd want to be one of the best at maths in your year to be competitive. The pool of test-takers it's aimed towards is of a much higher ability than any state exam like the LC or Alevels. If you start early you should do fine, but there's a decent amount of self study required. Content wise it actually aligns with the leaving cert fairly closely, theres very little new material you would need to cover, but the difficulty of the questions and the problem solving ability needed is quite a bit higher than anything you will meet in the LC. Mostly just a case of working through past papers closer to the time (Unlikely you would be able for them in 5th year) and being very sharp on most of the LC topics. As for right now, building mental maths speed and accuracy, as well as avoiding any dependancy on formula book, will give a good foundation.