Senior officials in Dublin spotted the issue directly after the Brexit vote. There would be at least two years of talking to agree the terms of Britain’s exit from the EU. Then there would a period of years as a new trade deal between Britain and the EU was hammered out. What would happen after Britain left, but before the new deal was done?
If there was no agreement to do otherwise, then Britain and Europe would – probably – impose tariffs and non-tariffs barriers to trade, based on World Trade Organisation policies. This is the "cliff edge" that British business leaders have been warning Theresa May about – and which she is prepared to try to avoid.
Avoiding the sudden imposition of trade barriers would suit Ireland, too. But will the rest of Europe play ball? It is unlikely they will, unless Britain too makes some concessions – perhaps on immigration. It would be in everyone’s economic interest to keep trade as free as possible while a new deal is worked out. But this isn’t about economics, it is now about politics. And if Britain wants a smooth transition it will, some way, surely have to pay for it.