Well the first question I'd ask is, did you pay rent or did you pay "rent"?
By which I mean, was what you were paying actually rent, or was it a contribution towards the cost of the groceries, household bills etc...
Obviously if you claim the tax credit, you are effectively flagging to Revenue the fact that your parents have an undeclared income source. Assuming they are in the tax net (i.e. working, earning and paying tax already) there's a good likelihood you'll cost them more money than you'll save yourself.
The relevance of the question I asked above is that if I were your parents, and Revenue came knocking, I'd simply say, nope that wasn't rent, it was money towards groceries, electricity, TV, gas etc... In which case Revenue may withdraw the credit they had granted you.
Bottom line is, someone has to lose out in this situation.