How much is your parent's house worth?
If your income allows it, UB will allow you to buy the house from your parents for €100k.
Your parents can then build the granny flat or lend you the money to build it.
For the first 5 years, you will pay your current rate +1%. After that , you will pay the SVR.
As you have a cheap tracker, would you consider renting the house in Dublin? The rent will cover the interest in full and probably most of the repayments.
You may be able to get a mortgage of €100k on your parents' home, depending on your income.