Along with divorce bill and EU citizens’ rights, Ireland is among top priorities in negotiations as UK prepares to leave bloc.

Why is the Irish border significant?

Post Brexit, the border between Northern Ireland and the Republic of Ireland will become the only land border between Britain and the European Union, raising the possibility – technically – of passport checks, customs checks and tariffs.

Passport checks are highly unlikely: the EU signalled in its negotiation guidelines that it would like to see a codified version of the 1920s common travel agreement – which means passport-free travel for Irish and British citizens between the two islands – included in a final deal.

However, it will be argued that if there are no passport checks, Ireland could become a back door for EU immigrants wishing to come to the UK. David Davis, the UK’s Brexit secretary, will have to find a way to guard against that.

Short of a Trump-style wall, the 310-mile border, which often zigzags through largely rural areas, would be impossible to police without a huge increase in resources.

To avoid customs checks, a workaround will have to be found as they are a requirement under EU law.

What is the border like at the moment?

Since the Good Friday agreement ended the Troubles almost 20 years ago, the border has become invisible.

The old British army checkpoints, security barriers and observation posts that became emblematic of the Troubles are long gone. The only clues to drivers that they have moved from one jurisdiction to the other are roadside speed-limit signs changing from kilometres (Ireland) to miles (Northern Ireland) and different coloured number plates.

It is a far cry from the height of the conflict, when anyone crossing the border was subjected to a routine ritual of questions – name, date of birth, address, reason for crossing the border.

Trade and services between the two sides now flourishes. Between 23,000 and 30,000 people commute across the border – a figure that discounts “frontier workers” such as community nurses or farmers who go back and forth across the border several times a day.

What have the two sides said about the issue?

The EU and the British and Irish governments are committed to the continuation of an invisible border.

“Negotiations should in particular aim to avoid the creation of a hard border on the island of Ireland,” the EU said in its negotiating guidelines. Theresa May has said she is also committed to a “frictionless, seamless border”.

Customs experts, however, have said this is fantasy.

Each border has two sides and while the British government can decide not to have customs checks, EU law requires Ireland to have them.

On the first day of talks in Brussels, a senior EU source said the Irish and British had been told that while there would be no need for “barbed wire and gun posts”, there would need to be checks.

“If you leave the single market, there must be checks,” the source said. “It might mean lorries turning off into a layby after going through the border but there will be checks.”

The former European commission customs lawyer, Michael Dux, drew gasps from MPs on the Northern Ireland select committee in February, as he told how every vehicle carrying goods worth more than €300 (£264) crossing from Ireland into Northern Ireland would, under EU law, be liable for checks.

Dux, who has 40 years experience in customs trade law, told how pets and horses would also need documentation to leave or enter the EU on the Irish border.

Physical checks of any kind will be strongly resisted by freight owners on both sides of the border.

Why is customs an issue on the border?

All-island trade in Ireland has flourished since peace, with production – particularly in food and drink – involving processing on either side of the border.

About a third of milk from cows in Northern Ireland is transported across the border for production into butter, cheese and infant formula. Industry representatives have warned that dairy farms would “go out of business” after Brexit if barriers to trade were erected.

HMRC statistics show Northern Ireland’s economy is highly dependent on exports to the EU, with 52% going to the European bloc including 38% to the Republic of Ireland.

Brexit will have a direct impact on Guinness, as the black stuff crosses the Irish border twice before being shipped from Dublin to Britain and beyond.

The drink is made at the St James’s Gate brewery in Dublin then pumped into tankers, known as “silver bullets”, and driven 90 miles to east Belfast, where it is canned and then sent back to Dublin port for onward distribution.

Does anyone have a solution?

One solution mooted is to move the border checks to ports and airports under a new unilateral agreement along the lines of the treaty of Le Touquet, which allows French border police to carry out immigration entry checks in Dover and British police to operate in Calais.

Technically, it is also possible that the customs checks on goods can be conducted electronically, with trusted trader status for regular cross-border freight and spot checks for standards.

The EU’s chief Brexit negotiator, Michel Barnier, visited border food businesses in May at the invitation of the Irish and told reporters that there was “always a way” of avoiding a hard border if there was “political will”.

An electronic check system could work, but tax chiefs say they are unlikely to be able to deliver this for a March 2019 deadline.

Are there other constitutional implications for Brexit?

Northern Irish politics is never smooth and both Sinn Féin and the Democratic Unionist party have made electoral gains as a result of Brexit.

The DUP, which is on the verge of being a powerbroker in Westminster, has warned that it does not want special status for Northern Ireland that would allow it to somehow remain in the EU.

Passport controls at Northern Irish airports and ports will also be anathema to unionists, who would see this as an attack on their Britishness, sources have suggested.

Brexit has re-energised the debate about a united Ireland, with Sinn Féin’s rising popularity fuelling calls by party leaders for a referendum on the issue.

The Good Friday agreement accommodates this, but a border poll is seen as unlikely at the moment.

What about the Republic’s trade with Britain?

The Dublin government is concerned about the potentially catastrophic impact Brexit will have on the Irish economy.

Britain remains Ireland’s biggest trading partner, with business between the two supporting 400,000 jobs and generating €60bn a year in trade in both directions.

A drop in the value of the pound since the EU referendum has made Irish exports to Britain less competitive, and some smaller businesses have already gone bust.

If tariffs are imposed after Brexit, then Irish goods, in the same way as other European goods, will become less competitive. There is talk of some businesses having to move to Northern Ireland or to places such as Wales in order to survive.