1) It is perfectly legal
2) If your fiancé and yourself buy a €350k house for, say, €290k.
You will both be regarded as getting a gift fo €30k each.
As this is well below the threshold for you, there are not CAT (inheritance tax) consequences for you other than it uses up part of your €335k threshold.
Your fiancé would have a €16,250 threshold, so would pay CAT at 33% on the €14k difference. (Or it might be €11k after a €3k exemption)
3) She could sell the house to you and there would be no CAT payable. Then when you get married you can pass half of it to your wife with no CAT liability.
4) Not sure what happens if you get married first and then she gifts the house to you.
Just to be clear, I don't think that this is a good idea at all.
- Although you both get on well with your mother, people change, and there might well be problems.
- What happens if you want to trade up or move because of your job?
- What happens if your mother needs to go into a nursing home?
By all means move into your mother's home and mind her. But you should both retain the flexibility to end this.
You do not need to own a home, if you are going to inherit your mother's home anyway.
Brendan