an interest free loan is a gift for Capital Acquisitions Tax purposes.
However, the "gift" is only the value of the interest foregone.
Every individual can get a gift of up to €3,000 for any person every year that is disregarded entirely for CAT purposes, so if the foregone interest were €6k per annum or less, this loan does not create any tax issues.
If it would be more than €6k per annum, then it will eat into the person's Tax Free Threshold. A person can get gifts/inheritances from their parents of up to €540k before they become subject to CAT (at 22%).
Obviously, if the loan were written off, the amount outstanding would become a gift.