You can't overpay a fixed rate without penalty.
But when they write to you with information about the reduced mortgage payment, you can tell them:
1) Please fix the rate
2) At the same time, reduce the remaining term of the mortgage so that the repayment remains the same.
If you think that you might want to overpay it even further during the three years at 2.55%, then you should ask AIB to split the mortgage into one part fixed and one part variable.
Then you can overpay the variable bit as and when you like.
Brendan