confused....you say you have three mortgages, one of which is for the new property that you are moving into. If you top up a mortgage, I believe that the total amount of the mortgage increases, but it doesnt split into two mortages.
You are correct in saying if you top up your existing mortgage with the """purpose""" of using the money as a deposit for your new PPR mortgage then you cannot claim interest releif on it.
If you rent out the old house in Dublin you can claim mortgage interest releif on the rental income.