You can give your sister a once off gift up to €50k and she would not have to pay any Capital Acquisition Tax on it.
So if the amount is less than €50k, then there are no tax implications.
I would think that an interest-free loan , repayable at her discretion would be aruably a gift. So if you give her a loan of €150k, it could be argued that the €100k is a gift and she would have to pay 25% CAT on it.
My gut feeling is that the Revenue would take a practical view of any such issue as long as it was not being done to evade tax. If you lent her money and she repays you, I don't think that the Revenue would worry too much about it. Especially in these tough business times.
I am assuming that she is a sole trader. If she operates through a company, it's a bit more complicated. You should be looking for security from the company e.g. a charge on the debts, so that you would get paid ahead of other creditors.
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