Proposed gift = €393,000
Group A (i.e. "parent to child") threshold = €332,084
Taxable gift (assuming no prior gifts from mother or father) = €60,916 (i.e. €393,000 less €332,084)
CAT on above gift = €15,229 (i.e. 25% of €60,916)
But mother's discharging daughter's CAT liability, therefore:
Total deemed taxable gift = €76,145 (i.e. €60,916 plus €15,229)
CAT payable = €19,036 (i.e. 25% of €76,145)
The "€3,000 exemption" may of some benefit here. Basically, an individual can receive a €3,000 gift every year from any number of other individuals (i.e. during 2011 I could receive €3,000 from the OP, €3,000 from Brendan, €3,000 from any number of other people and then I could receive the same from them all during 2012 et seq).
A further high level thought - If (for example) the gift is for you guys to buy a home together, and your "mother in law" and girlfriend would be happy to do so, gifting 5% of the proposed gift directly to you might make sense as it would use up your Group C (i.e. "stranger") threshold. You may not want to do this though and your girlfriend and her family may not want to either. Just a high level thought rather than a recommendation and possibly another example of when tax considerations shouldn't drive everything else!