History is a funny business. When most of us think about a united Ireland (which is not very often), we imagine it as a grandiose final scene from an epic movie. But history now turns on small and dull matters. Just now, if you want to think about the future of Ireland as a whole and of its relationship with Britain, you have to think about boring stuff – stuff like the functionality of UK border controls. It is as if the GPO in 1916 had really been all about the British postal system.

In 2016, more than 310 million people and nearly 500 million tonnes of freight crossed the UK’s borders. If this continues to happen in a “frictionless” way after Brexit, the disturbances to the status quo in Ireland will be limited. If it doesn’t, hang on to your hats. Frictionless trade is the only condition under which Brexit can happen without inflicting a hard border on Ireland. It is almost certainly a political impossibility if the UK leaves the customs union. But even if it could somehow be agreed in principle, there is another enormous obstacle: the actual capacity of the British to handle it.

Glowing endorsement On Friday, after Theresa May’s big set-piece speech on Brexit, the DUP leader Arlene Foster issued a glowing endorsement. She referred back to a paper issued by the UK government last August: “Those proposals can ensure there is no hard border between Northern Ireland and the Republic of Ireland after we exit the EU.” Foster recognises how much unionism is staking on that document and on the ability of the UK’s bureaucracies to deploy technology to take the sting out of the potentially toxic irritant of the Irish Border.

The cancellation of the contract led to a legal settlement for another £150 million (€168 million). The Home Office then spent another £303 million (€340 million) on a new programme, bringing expenditure to £830 million (€940 million). In 2015, the National Audit Office reported that all of this expenditure “has failed, so far, to deliver the full vision” of what was supposed to be achieved. The current date for completion of the programme is 2019. The whole thing will have taken a mere 16 years. On the same timescale, the new post-Brexit systems on which the future of Ireland may hinge would be delivered in 2035.

In 2015, 55 million UK customs declarations were made by 141,000 traders. Once Brexit happens, that will increase fivefold to 255 million. Leaving aside all the issues of political principle, this is the vast logistical challenge that will have to be dealt with if May and Foster are to get the Brexit they want.

Untested approach This brings us back to the paper on customs the British produced back in August and that Foster hailed last Friday as “innovative”. Actually, the words that paper used about its own proposals are “an innovative and untested approach that would take time to develop and implement”, which is management speak for “nobody has done this stuff ever before, we have no evidence that it would work and we have only the vaguest outline ideas about it”. In relation to the idea of a “customs partnership” (as opposed to a customs union) with the European Union, which May embraced on Friday, the August paper says: “This is of course unprecedented as an approach and could be challenging to implement”.

The one big idea on which the DUP places much of its hopes is the “trusted trader” – companies whom the customs authorities get to know and trust to fill out all their own declarations honestly. There is nothing, of course, to stop the UK from using this system right now for trade outside the EU. So I wondered how many “trusted traders” the UK has designated so far. Of the 141,000 companies using its systems, a grand total of 604 are trusted traders. That’s 0.43 per cent. Which prompts an obvious question – if this is such a great solution, why are more than 99 per cent of UK companies not using it? Why are UK authorities not currently taking any steps to promote it?