If reunification happens then it's in the interests of all neighbours and trading partners that the new Ireland is both politically and economically stable and successful. In the same way that a hard Brexit would have been bad for Ireland and much of the EU.
The annual UK subvention to NI is estimated at between £3bn and £10bn depending on how it's calculated. A future UK government may well consider paying a portion of that for a defined period to get NI off the books long term.
Or maybe they would make it as difficult as possible to deter Scotland from breaking free...