Problem is the Deeds
Stamp Duty wont be an issue
She will be liable for Gift tax ( Current value - mortgage ) = Gift value
Then minus €23,901 allowance from Gift value and *20% and thats what she will pay if on deeds.
Can I suggest putting her on the mortgage only. Then she has the security that you cannot sell without her, she gets what she contributes to mortgage. Then at later date if you get married you can put her on deed without shelling out to revenue, as their are no gift tax between spouses.