Are you talking about switching to a new lender or to a different product within Bank of Ireland?
Switching to a new product within BoI
As PGF says, make sure to avail of any cash back at the end of the term.
Otherwise, check about reducing the balance now and see if there is a break fee. You don't need to wait until the end of the term.
BoI's products don't reward people much for lower Loan to Values, but check this out.
Switching to a new lender
You should not be switching to a cash back lender, but if you are, then borrowing the maximum is the right strategy. Take out a variable rate initially, pay down the mortgage as soon as soon as you get the cash back and then fix to get the best rate.
The best value lenders - e.g. Avant offer much lower rates for lower Loan to Values, so get the house valued and try to get the LTV below 60%.
Some lenders have lower rates for larger mortgages. But they are not usually competitive overall.