There was an interesting article in The Irish Times today about the consequences for Ireland when the (r-)UK leaves the EU’s customs union:
Even if there were a free-trade agreement, allowing free movement of goods between the UK and the EU, this will not apply to imports from countries outside the EU. Thus border controls will be essential to ensure that imports from third countries, such as China or India, comply with EU regulations.
This will have huge implications for the retail sector, much of which currently operates on a British Isles basis. Goods travel from warehouses in the UK to the Republic without problem. After Brexit, this will require new bureaucracy and customs duties, entailing a major increase in cost.
That could raise prices significantly for Irish consumers, posing serious competitiveness problems for the wider economy. Because of the small size of the Irish retail market, going it alone is a high-cost option.
Although they don't mention it, this is likely to be a much bigger problem than most people realise because of the way modern businesses depend on just-in-time manufacturing and low stock levels. What this means is that shops tend to get new deliveries all the time instead of having a lot of stock, and this will lead to huge problems if the over-night deliveries sometimes get disrupted by customs checks.
It's possible some companies will start treating Ireland as part of their French operation rather their British one (for instance, Kellogg's might prefer to sell the products made for the French market in Ireland instead of the British ones to avoid the customs checks). That would definitely make Ireland feel less British over time.
From a Scottish perspective, it means that if we leave the UK at the same time as Brexit in order to remain in the EU, and if the rUK proceeds with the harmful policy of leaving the EU's Customs Union, retailers are likely to start treating Scotland and Ireland as one market (which again might been seen as a subdivision of the Scandinavian or the French one), whereas the rUK will be seen as a rather distinct one. It would make the products in Scottish and Irish shops more similar over time, and less similar to the ones found in England and Wales.
It's worth bearing in mind that the EU's Customs Union is almost ten times as big as the rUK's one, so although it will be annoying to lose some of the English and Welsh products in the supermarkets, the consequences for Scotland will be much worse if we leave the EU together with the rUK.
I'd much rather Westminster decided to remain in the EU's Customs Union, but if they really are hell-bent on leaving it, it's yet another argument why Scotland should become independent within the EU.