If your father is now saying he doesn't want it back, it has become a gift (well unless you're the Taoiseach!). However, the amount you can receive from your parents before paying tax is €496,824 so you probably have no tax payable.
it's not quite like that. He is not saying he doesn't want it back. It is still a loan and we are still within the period that we expected me to hold on to it.
Well, maybe you would be best to put this loan in writing. Generally, a loan does not get written off because someone dies, it is repayable to their estate and so would form part of your father's assets (to be distributed according to his will).
However, if he wishes it to be written off on his death, it will then become an inheritance to you.
So, in either case, if it is not repaid, it will eat into your tax free threshold for CAT.