Even just changing the term to 30 years with AIB and no change of rate would cost you €150 less per month. Not saying you shouldn't switch to UB, just that you should know in advance which parts of the change -- lower rate, longer term -- have which effects. Lengthening the term could be a false economy if you don't need it, as you will repay a higher total in the long run. Overpaying will generally just have the effect of reducing the term, reversing the effect of your switch to a longer term, but perhaps giving you more options in terms of varying your repayment when you want.
It's all workable outable on a mortgage calculator or spreadsheet -- if you post the principal amounts, rates and terms I'm sure some kind person here might help.