If account is taken of an outstanding mortgage ( say for arguments sake: E250K ) then 340K less 250K is 90K of which 45K is the actual equity to be paid to the other person. That is under 127K so it is possible that stamp duty will not be due.
There are conflicting views as to whether this method of calculation is acceptable to Revenue - I've successfully battled with them before on it. In a case where a lender joins in a Deed and releases one person from the mortgage, consents to the transfer of the property into the other named owners name and that person becomes solely responsible for the mortgage, on the above figures, there would be no stamp duty liability. So why should it be different if the person to acquire takes out a new mortgage?
Its worth getting proper advice on this.
mf