Loan vs Gift

If your parents give you an open loan but do not look for it back to Revenue consider it a gift?

Well Revenue would look at the 'facts'.
It seems clear to me that the intention was that this was a loan. So it either was or it wasn't. How the parents were going to get it back, is another matter. If they write it off, then its a gift.
So the only people that know the facts are you, spouse and parents.

Basically depending on who is gifting to you, decides the which of the three 'classes' the gift is in. These classes have different 'sizes'. The one from the parents is that class that has the highest 'threshold'. Unfortunately that one gift means it has at the same time eliminated the thresholds in the other classes. Simply this : If you now got a gift/inheritance from an old neighbour, then this would now be fully taxable. (This is trying to summarise it simply!)

Do you need to tell Rev if you receive a Loan

No why would they need to know about the loan?
The issue seems to me whether this ends up as gift or not.
As that will affect any other gifts.

but what if the loan is interest free?