The stamp duty, if payable, will be payable on half the market value of the house less half the mortgage. So take the value of the house, divide in two and take off half the outstanding mortgage and that is the value you are acquiring and on which you may or may not have to pay stamp duty on. If the 80K is an arbitrary figure, its not relevant. I've done a few of these recently and even Revenue are confused.
Plus if you were both first time buyers at the time and have not acquired any other property in the meantime, you may still be considered a f.t.b.
mf