It is unfortunate that the UK government decided to trigger Article 50, without first working out, around the Cabinet table, what sort of relationship the UK could reasonably expect to have with its neighbours after it had left the EU.

It is true that Theresa May presented a wishlist in her Lancaster House speech. But this list was, and is, impossible to achieve because it took no account of World Trade Organisation (WTO) rules, and of the fact that commerce can only be free, if the rules governing it remain reasonably uniform.

May said that on the day it leaves the EU, the UK will retain all the then existing EU rules for goods and services, but would be free to change them, by Act of Parliament or by legal reinterpretation from then on. This implies a gradual, surreptitious, hardening of the border in Ireland, as UK standards begin to diverge from EU standards.

To the extent that the UK diverges from EU standards, UK businesses will have to apply two sets of standards, one for the UK market, and another for the 45% of UK exports that go to the EU.

Once the UK has left the EU, goods coming into the EU (including into Ireland), from the UK will also be subject to checking under “Rules of Origin” requirements, to see that they do not contain impermissible non UK content. For example, there might have to be checks that UK beef burgers do not contain Brazilian beef. These “Rules of Origin” checks will involve bureaucracy, which will be especially onerous for small firms.

One study estimated that the need to apply “Rules of Origin” checks could reduce trade volumes by 9%. The EU-Canada Trade Agreement has 100 pages on “Rules of Origin” alone.

As far as imports into the EU at Dundalk or Lifford are concerned, it will be Irish, not UK, officials who will have the distasteful job of enforcing the border. Irish officials will have to check whether EU safety, sanitary, origin and other rules have been met. This is something being imposed on us, as an EU member, by a UK decision.

It will not be done cheaply. A House of Lords committee said that “electronic systems are not available to accurately record crossborder movements of goods”. Even if a light or random system of checking at the border or at ports is imposed, the biggest costs will have to be met by businesses.

The preparation of all the extra compliance documentation will deter many smaller firms from exporting at all. For example, it has been estimated that the number of customs declarations that UK firms will have to prepare and present will jump from 90m a year to 390m once the UK leaves the EU. Irish firms will have a similar extra burden in their dealings with the UK.

The focus on the Irish border should not deflect attention from the effect of Brexit on east-west trade. Indigenously owned Irish exporters rely disproportionately on the UK market, and their customers are predominantly on the island of Britain, rather than in Northern Ireland. The bulk of Irish exports to continental Europe transit through Britain.

Customs union

Theresa May was remarkably unclear, in her Lancaster House speech, about the sort of relationship she wanted with the EU customs union.She wanted bits of it, but not all of it. This would run into immediate difficulties with the WTO. The WTO works on the basis of non-discrimination, or the so called “most favoured nation” principle. This means that any concessions the EU customs union might grant to the UK, as a non-EU member, would have to be extended by the EU to all of the EU customs union’s trading partners. That is unless the special UK concessions cover “substantially all” trade between the UK and the customs union. The UK will have to be either ‘‘substantially in”, or “substantially out”, of the customs union. Which does May want?

Many suspect the UK wants to leave the customs union so it can revert to the cheap food policy that it had before it joined the common market. To prevent the undermining of the EU Common Agricultural Policy that would flow from that, Ireland would then be obliged to collect the EU common external tariff on products such as beef, milk, lamb, confectionery and other food products crossing our border, or arriving at our ports, from the UK.

DUP and Sinn Féin

Farmers and investors in the food industry need to know what sort of agricultural policy the UK will choose after it leaves the EU.

The DUP, which supported Brexit, and which is now dispensing gratuitous advice to the Irish Government on EU matters, should tell us exactly what sort of UK agriculture and food policy it expects post-Brexit. It is now in a good position to get an answer to that question from the UK government.

But the DUP is not the only party that needs to examine its position.

In September, the UK Parliament will begin debate on the European Union Withdrawal Bill, which will allow for UK standards to diverge from existing EU standards, thereby deepening the Irish border.

Thanks to the abstentionist policy of Sinn Féin, there will be no Irish nationalist MPs in that debate able to put forward amendments to mitigate the hardening of the border. Until this year, three SDLP MPs would have been there, but now, when their presence was never more necessary, the voters have replaced them with Sinn Féin MPs, who will draw their salaries, but will stay away when decisions have to be made, leaving the field to the DUP.

Sinn Féin should remember that the Irish people, on both sides of the border, accepted the Good Friday Agreement in a Referendum in 1998, and that removes any “nationalist” argument Sinn Féin might have had for not taking its seats. If Sinn Féin can shake hands with the Queen, if it can take its seats in Stormont, it can take its seats in Westminster. There is work for the party to do there now.