@GingerOHEasy way to fix this, just give Ireland the northern counties back, they are willing to alter their constitution to legalize marriage equality.
And, as we're all democratic people; let them have a referendum to decide if they want to join the ROI. They have always been against it.
As there is, yet, to be Gay Marriage in the ROI (only civil partnership, as in UK although gay marriage has got past the first vote in the UK commons, comfortably), your proposal would not sort anything. As you observe, the ROI will also have to change its Constitution, legislation will not be enough.
I believe in both gay marriage and a single united Ireland - but I believe in self-determination more.