The CAT threshold between 3rd parties is € 23,908 for cumulative gifts since Dec 1, 1999. There is also a small gift exemption of € 3,000 per annum. Thus the calculation would be
Gift 40,000
less small gift exemption (3,000)
=chargeable gift 37,000
less threshold (23,908)
=taxable gift 13,902
tax at 20% = 2,618
assuming that you have not received any other gift from a 3rd party since 1999.
Of course, she could give you 37,000 this year and 3,000 next year thus reducing the tax bill still more.