It seems to me that the contract is pretty clear. Interest only for x years followed by capital and repayments.
If you want to renegotiate the terms, then both sides are free to do so. If you don't pay your capital and interest you are in breach of your contract and will go into arrears. Check the mortgage contract to see if there are any penalty charges for going into arrears.
Some people are challenging PTSB's right to switch them to interest and capital in the High Court. YOu should find a thread on this elsewhere in this forum.