This seems to be a fundamental misconception many of you Brits espouse. Here is the crux of the matter. The UK government has stated, regarding this matter, that following Brexit vote, it will not only leave the EU, but as well the SM and CU. It has (HMG), also, stated, that there will be no hard border between RoI and NI and that the GFA is respected. These two red lines are mutually incompatible -- in the real world. The EU, and hence the Irish government, first, proposed that if NI would stay in the SM and CU -- laying the border on the sea -- and would solve said issue. This was rejected by London. Sovereignty or some such bumper-sticker was shouted. Then the EU and, again, the Irish told the UK government to stop the BS and come up with an acceptable solution.