To be mutually beneficial both parties would have to be equal.The point is that when you said
You were explaining why NI should be in a close and mutually beneficial relationship with RoI, and why UK should be in a close and mutually beneficial relationship with the rest of Eufope.
Under this EU regime N.Ireland would be subject to rules over which they have no veto.
There will be no political representation in Europe for N.I.but it will still be subject to the European court of justice.
How can living in an economic colony of the EU be beneficial to anyone.