However, if May insists on leaving the EU customs union, there will have to be a "hard" border - and if there is, says Ireland's Prime Minister Leo Varadkar, he will veto any negotiations between the EU and the United Kingdom on a free trade deal after it leaves the Union.
They took to the streets to ask random British people about Brexit's impact on Ireland, and they asked them to draw a line on a blank map of Ireland showing where the Republic ends and the North begins. "If there is no border we might as well do away with the internal market".
He said: "The one thing that the European Union doesn't want is a divergence in regulatory standards between the north and south of Ireland, if that was to happen then you would need a hard border".
In principle, Britain flouncing out of the European Union shouldn't hurt anybody except the British themselves, but the UK's Irish border is a nightmare.
"I think our neighbour has acted disgracefully", said Mr Paisley, who is separately under investigation by the United Kingdom parliamentary watchdog over claims he failed to declare holidays to Sri Lanka.
The UK environment secretary, Michael Gove, has said that while he would like to see the British regulatory regime diverge from the rest of the European Union, animal welfare standards are likely to be strengthened, not weakened. Instead came the Easter Rebellion of 1916, the Irish War of Independence, the partition of the island between the independent Republic and Northern Ireland (part of the UK), the Irish Civil War, and three decades of terrorist war in Northern Ireland that only ended 20 years ago.
There is concern on both sides that if there is regulatory divergence, cross-border trade and social cohesion that has flourished since the Good Friday agreement will be jeopardised.
The UK would be allowed some restrictions on free movement.
"The truth is that Northern Ireland is different from other parts of the United Kingdom because we agree to do things on an all-Ireland basis - in other words it's different to Wales and Scotland, and that's just a pragmatic difference".
The paper quoted sources in Dublin as saying that there had been "movement" on the issue and confidence was growing that agreement could be reached in time for next month's summit in Brussels.
"The southern Irish have to lump it, basically, you can't always have what you want in life". "The question is how detailed should this be and how it should be formulated".