Are my calculations correct for someone in this hypothetical situation.
Let's say someone has a BTL mortgage with BoI at the 5.8% rate for a LTV of > 75%.
That person would be allowed to write 75% of 5.8% off against rental income for tax purposes, or 4.35%. At 41% tax, 4% PRSI and 7% USC, this would result in a deduction in tax/PRSI/USC payable of 2.26% of total borrowings (4.35% * 52%). Therefore, the net cost of borrowings could be considered to be 5.8% - 2.26% = 3.54%.
Therefore, if that person had a residential mortgage rate of less than 3.54%, they're actually better overpaying the BTL mortgage as opposed to the residential mortgage.