How much will you get in rent?
How much are the mortgage repayments?
How much of the repayments comprises mortgage interest?
The answers to those 3 questions will determine, to a large extent, how much you will be paying in tax and/or making up a shortfall in mortgage repayments.
I don't know what the situation is with your existing property but if the bank were to allow you to go interest-only for a set period (say 3 years) then should reduce this burden considerably.
eg - suppose monthly repayments are currently €1,200 per month, €900 of which is interest
You rent for €1,000 per month
Rental Income = €12,000
Allowable Interest = €8,100 (€900 x 12 x 75%)
Other expenses = €1,000 (house insurance, life assurance, repairs etc)
Capital Allowances = €1,000 (estimate)
Rental profit = €1,900
Taxed at 47% = €893
You also have to make up the shortfall of €200 per month so the total cost to you is €3,293 per year, in addition to the renat payable on the new home
If you can get your bank to go interest only, the rental computation stays the same except this time you DON'T have to make up the €200 per month because you are actually making a profit on the rent vs mortgage aspect (ignoring other expenses)
There are other issues to bear in mind though, one of which is if you are on a tracker rate on your existing mortgage, the bank may take the tracker off you as you no longer live there as your PPR.