I have a plan,
Clearly we don't want to lose the economic benefits of being in the customs union and single market, that would be economic suicide, also we don't want to break up the United Kingdom and risk ripping up the Good Friday Agreement - who wants to re-start a civil war? On the other hand we don't want to be a "vassal state" taking rules without any say in them and having to accept trade deals negotiated on our behalf by third parties.
How about an arrangement whereby we participate in the customs union and single market, enjoy open borders and have share in the say of the governance of these things proportionate to our size and wealth? Sounds like the best of both worlds.
Oh hang on...