If you can make overpayments, make them against the active mortgage.
This is really important. A split mortgage is really two separate loans. One on which you are charged interest and the other where there is no interest. It is always right to pay off the loan with the higher interest rate first as that will save you interest in future.
Let's take an example
Aged 50
Remaining term on the mortgage: 20 years
Active mortgage €100,000 @3% interest - repayment €550 per month
Warehouse €50,000 @ 0% interest and no repayments.
You have the capacity to pay an additional € 300 a month.
1) Increase the repayments on your active mortgage to €850 per month
2) You will clear the active portion of the mortgage in 12 years.
3) You can then start working on the warehouse.
If you continue to pay €850 per month and the bank still does not charge interest on the warehouse, you will have
the warehouse cleared in 5 years.
So you will be mortgage-free after 17 years in total.
4) There is a risk that if the bank sees that you can pay more, they might move some money from the warehouse to the active part. So instead of increasing your repayments to €850 per month, maybe save the €300 into another account and every so often pay off a lump-sum of maybe €2,000 or €3,000. You do save more by paying the money in earlier, but it's not that significant.
If you have an AIB split mortgage, you can get a discount for clearing the warehouse, so that makes more sense than paying down the active part.
https://www.askaboutmoney.com/threa...ouse-on-my-split-mortgage.227127/post-1768399
If it's over 10 years since you got the AIB split, the discount is no longer available, so pay off the active part of the mortgage.