IT MIGHT just be Ireland’s most famous product. And indeed, the Guinness that is brewed at the St James’s Gate brewery in Dublin is shipped to Europe and across the Atlantic. But first the stout is transported north, in tankers that have become known as “silver bullets”, to be canned and bottled in east Belfast before returning to Dublin for export.

Diageo, the multinational company that owns Guinness, says that its silver bullets make some 13,000 border crossings a year. The company estimates that even a short delay of 30 minutes to an hour for customs checks would add about €100 ($115) to the expense of each trip, costing it some €1.3m a year. If that happened, the price of Guinness might have to rise.

Guinness is not alone. The abolition of customs controls in 1993 and of security checks after the Good Friday Agreement in 1998 has led to the creation of what is in effect an all-island economy, with supply chains criss-crossing the border. Bilateral trade between Ireland and the United Kingdom is now worth over €1bn a week. Most of that trade goes over the Irish Sea, but a fair amount crosses the border with Northern Ireland, especially in the agri-food sector. The UK exports more food to Ireland than to Canada, China, Japan, Russia, Saudi Arabia and South Korea combined.

Thus, besides Guinness, it is said that the ingredients of Bailey’s Irish Cream, another drink owned by Diageo, travel across the border three times before being exported in bottles. Roughly a third of the milk from cows in Northern Ireland goes south for processing, while much of Ireland’s cheese goes in the opposite direction. Sheep and cows are frequently driven across the border for slaughtering. Economies of scale mean that it does not make any sense to have two processing plants on the island for most foods; that could be threatened by a hard border.

To complicate matters, food and drink have always been among the most sensitive products in the EU. Michel Barnier, the European Commission’s Brexit negotiator, has said flatly that after Brexit 100% of imports of animals and animal products from Britain will be subject to border controls. If a putative free-trade deal with America were ever to come about, Britain might well allow the import of chlorine-washed chicken and genetically modified foods. Given the public’s hostility to these things, nobody in Brussels could possibly risk letting them cross into the EU by the back door of the Irish border.

Correction (July 13th): An earlier version of this story wrongly stated that all Guinness is brewed in Dublin. In fact there are several Guinness breweries outside Ireland. This has been corrected.