A little difficult to work out your question but I think what you are asking for is is it better to borrow €150k now and pay a lump sum off the mortgage in 2022 or is it better to take €20k now (since you would be penalised that I think) and start with a lower mortgage.
For the sake of convenience I am going to assume the same fixed interest rate applies to both and will do so for 5 years. That is not a very safe assumption but it makes it difficult to calculate otherwise!
I would suggest using an online calculator such as this one to play with the figures yourself as you will be better able to provide useful input to it:
https://www.drcalculator.com/mortgage/ie/
Using that with the following parameters
Principal: €150,000
Interest: 3.1%
Term: 25 years
gives an overall cost of €215,743 (€65,743 in interest over the 25 years)
and a monthly repayment of €719
Principal: €130,000
Interest: 3.1%
Term: 25 years
gives an overall cost of €186,977 (€56,977 in interest over the 25 years)
and a monthly repayment of €623
So based on that the cost of borrowing the additional 20k works out to €8766 in interest and nearly €100pm on your repayments.
Repaying a lump sum of €30k in 60 months (5 years) time would provide a considerable saving on the overall cost of a €150k mortgage. Plugging that number into the calculator gives you a saving on interest of €20,859.
Based on that it does look like a good idea to hold onto it.
However...
If you can afford to comfortably pay €719 pm for the higher mortgage amount to hang onto your bond then the question is why pay only €623 for the lower mortgage? If you were to go for a 20 year term at that rate instead of 25 you would make a similar saving. Alternatively if pre-payment was an option you could keep the term but still benefit from accelerating your balance reduction.
The other point to consider is whether you would be wiping all of your savings doing this - you may want to place a value on the bird in the hand, having some savings is generally preferable.