If your tracker stayed at 1.25% for the next 20 years (which it won't), you would pay €23k in interest.
If you went with Avant's 7-year fixed rate at 1.95%, and and they offered that rate for the following 13 years (which they won't), you'd pay €37k in interest over the 20 years.
If Avant's 10-year fixed rate of 2.1% lasted for 20 years, you'd pay €40k in interest.
If you went with Finance Ireland's 20-year fixed rate at 2.5%, you would pay
€48k in interest (and your monthly repayment would increase by €105). They are apparently offering €1,500 cashback to people who switch to them before the end of March but I can't find much info about this.
The 20-year fixed rate at 2.5% seems like expensive insurance to me, but ultimately it comes down to your risk appetite – how much extra interest are you willing to pay for the security of a longer fixed rate?
If you think you'll be in a position to make fairly large overpayments in the next few years, then fixing for 20 years kind of seems like a waste, because if you make overpayments while keeping your monthly payment the same, you'll clear your mortgage in less than 20 years.