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.