I've just switched mortgage and come up against this as the house and mortgage were originally in my name alone but I've got married a few years past.
The issues you'll find are
1) Bank doesn't like joint mortgage when one person not on the deeds. They were ok with us though as we are married and wife has a right in law regarding the family home. I'd imagine if you are not married this would be a bigger issue.
2) If you do manage to get on the mortgage you will need to get independent legal advice (Cost varies 100-150 euro, not paid by bank) as you will effectively be responsible for a mortgage for a house for which your name is not on the deeds.
3) You could get you name on the deeds and then there will be no problem. Once again you need to pay for this yourself as bank will not cover this. Cost about 230 euro.
We went with Halifax and used their legal services through First American. First America will organise the additional legal items in 2) or 3) but as I say the additional costs need to be paid by yourself.
Not sure if there is really an upside to you having your name on deeds or mortgage. Once married the 'family home' is protected in law, even if your wifes (to be) name is on the deeds and mortgage she could not remortgage the house without your permission ... that's what we found.
I originally intended to keep things simple and get the mortgage in my name alone as the original mortgage was, but my wife would have been required to have got independent legal advice here too as I would be remortgaging the family home.
If I were in your shoes I'd just let her proceed with the application in her name alone. Less complicated and no additional cost.