Nope, there are only 2.There are 3 protagonists in this matter.
1) UK
2) ROI
3) European Union
A customs union, border checks, and above all tariffs, are only in the interests of one of the above.
I'll let you guess which one....
EU and Ireland are on the same side - a fact which seems lost on most pro-Brexit people, or they choose to ignore it.
This is what happens when you leave the EU - you are on your own.