"If i can prove they paid for it all along is everyting ok tax wise?"
Yes and no.
Go and talk to a good tax advisor - someone who is good with property taxes. You have all sorts of issues here - CGT, CAT and stamp duty being the primary ones.
Talk to your solicitor. Check precisely who is on the deeds i.e. who is the registered owner. I cannot understand why your parents were not on the deeds and the mortgage? Unless, someone decided to keep them off title for some obscure reason? Is there any possibility that a Deed of Trust was executed at the same time?
You need to work out the precise costs in transferring ownership to your parents. It may not make any sense to to it - it might be better, tax wise, to transfer a share to them or grant them an exclusive right of residence.
mf