Hi Trust
If you charge 3%, or more, interest on the loan, there are no tax consequences for her, except that you will have to pay income tax on the interest received.
If you give it interest-free, in theory, it's subject to Capital Acquisitions Tax on the benefit received. Say €80,000 @3% =€2,400 which is below the €3,000 annual exemption. So no problem there. (In any event, Revenue are not interested in such transactions.)
You do not need to draw up a very formal contract. But you and she need to be clear in writing what you are doing. A short letter to her saying "Dear Aunt Nell, I enclose a cheque for €80,000 as a loan repayable when you sell your own house." Ideally, but not essentially, the closest relatives e.g. the children or the sister of the woman should know what you are doing, just to make sure that there is no dispute if anything happens before she sells her house. But if you or she wants to keep it private, then there is no need to do this.
Thirsty's advice might make some sense in the case of helping out someone in financial difficulty who is unlikely to be able to repay the loan or if it is for a very small amount. It makes absolutely no sense in this situation. If you give her a gift, she will be subject to CAT on it. If she gifts it back to you, you will be subject to CAT on it.
You are effectively giving her a bridging loan. Go ahead with it. Well done. That is what family is for.
Brendan