Let's be clear about what you want here.
You have a mortgage of €200k with 20 years remaining which results in a repayment of €1,100 a month.
You want the term to remain the same, but you want to overpay by €200 a month. That seems very complicated to me.
If you paid off a lump sum of €30k, I could see how BoI would either reduce the term or reduce the repayment. But they can't be expected to do that for you every month for €200.
You could just continue to pay the extra €200. They will credit it as an overpayment and not reduce the term or the repayment unless you ask them.
Brendan