Hi misterfox
Unless you have a tracker mortgage and can keep it on the new property, there is no advantage in moving the mortgage to a new property. (You can't do so anyway - they always issue a new mortgage).
You will need to read this thread to understand what is happening
A guide to mortgage repayment calculations
If you borrow €100,000 at 3%, you will be charged 3% interest or €3,000 in the first year.
If you make no capital repayments, you will be charged €3,000 again the next year.
If you pay €5,000 every year, then you knock €2,000 off the mortgage, so in the second year, you will be charged €2,940 (€98,000@3%) in the second year.