In normal times, there would be a straightforward answer to this question.
But the market is so dysfunctional at the moment in that those lenders with the best fixed rate deals often have very high variable rates. The best fixed rate deal on offer for a 90% LTV mortgage is probably the three year Bank of Ireland mortgage at 3.2% with 2% cash back. But their variable rate is 4.5%.
So it seems wrong to have any part of your mortgage variable at 4.5%.
I understand that Bank of Ireland allows you to repay 10% of the capital without penalty. (Can anyone confirm this?)
So it would be wrong to go 90% fixed and 10% variable when you can pay the 10% anyway.
Alternatively, you could take your chances and just pay the early repayment fee. It seems that anyone fixing now, August 2017, will have a very low early repayment fee.
Yet another approach, would be to fix for just one year at 3.35% and, if early repayment is too expensive, then save up your "overpayments" and pay down the mortgage when the year is up and then fix again for a year.
Or you could just go with KBC and fix at 2.9%. But even their variable rate is 3.5%.
For lower LTVs, the difference between fixed rates and variable rates may not be as stark.
Brendan