“No UK prime minister could ever agree to it,” Theresa May told the House of Commons in 2018.

By August 2020, her successor, Boris Johnson, was vowing: “There will be no border down the Irish Sea – over my dead body.”

Yet a customs and regulatory frontier between Northern Ireland and Great Britain (England, Wales and Scotland) is exactly what he has agreed to with the EU. The alternative would be a hard border with customs checks between the Republic of Ireland and the north – which is a crucial “red line” for European negotiators.

As a result, from the end of the Brexit transition phase, Northern Ireland will continue to function largely as though it is still in the European Union – which will perhaps please the 56 per cent of voters who chose Remain in the 2016.

As a result of the UK government’s decisions, there are significant changes for individual travellers. These are the key consequences.

No, except in two narrow and unusual circumstances.

First, if you are travelling from Great Britain to Northern Ireland with more than €10,000 (£9,100) in cash, cheques or banker’s drafts you must declare the fact in advance, explaining where the money came from, to whom it belongs and how it will be used. 

Second, many travellers (in non-Covid times) travel from England and Wales to Northern Ireland via the republic. Ireland is in the Common Travel Area and therefore no passport is legally necessary. But airlines may insist on seeing one as identification; Ryanair routinely does.

Incidentally, motorists travelling to Northern Ireland via the republic, for example Swansea-Holyhead-Dublin-Belfast, will need a Green Card from their insurer for the drive through Ireland.

Yes, when travelling from Great Britain to Northern Ireland (effectively going into the European Union for customs purposes), though not in the opposite direction.

For individual travellers, rather than freight consignments, the checks are likely to be light touch. Most meat, milk and products made in Great Britain are banned.

The government gives the example of “cheeses with added herbs, sausages with garlic [and] yogurts with added fruit” as items that cannot be carried from Great Britain to the EU without an export health certificate. So finish your picnic before arriving in Northern Ireland.

No restrictions apply when travelling from Northern Ireland to Great Britain.

No additional customs checks or restrictions will apply, whether you are travelling from Belfast to Barcelona or from Dundalk (in the republic) to Newry (in the north).

But anyone seeking to exploit the absence of customs checks between the European Union and Northern Ireland and from there to Great Britain is warned against “Belfast bootlegging”.

“We are clear that we will continue to tackle smuggling and illegal activity in all circumstances,” said a UK government spokesperson.

Anyone who lives in Great Britain who wants to take a pet to Northern Ireland must take a number of steps in advance.

The cat, dog or ferret must have an animal health certificate obtained 10 days in advance and, for dogs, tapeworm treatment administered between one and five days of entering Northern Ireland.

This must be carried out on every visit.

Cats, dogs and ferrets in Northern Ireland, are, like the human population, in an advantageous position in terms of border checks and ease of movement.

Northern Ireland will remain in the EU pet passport scheme. In addition there will no restrictions for pet owners who live in the country returning from Great Britain.