Among other things, the European Union is a customs union. As UCD’s Barrett explains: “A customs union has two elements to it. One is the abolition of customs controls between the states that are a member of the customs union, for example between Ireland and the UK [including Northern Ireland], between Ireland and France, between Ireland and Germany.”

With regard to the second element, he continued: “In exchange for that, though, you erect a tariff wall around the customs union. A customs union involves free movement for all of the goods that are in the customs union, whether because they are produced in the European Union or because they have been produced outside the EU, we’ll say in South Korea or China or wherever, and the non-EU producers had paid whatever the common customs tariff is, to come over that wall.”

If, as a result of Brexit, the United Kingdom leaves the EU customs union, Northern Ireland would go too, creating a situation where Northern Ireland wouldn’t be in the EU customs union, but the Republic of Ireland would. This would result in a hard border between the two, and the inevitable return of border posts. Leo Varadkar, the Irish prime minister, has threatened to block progress on Brexit negotiations unless May’s government can guarantee that there will be no hard border.

Queen’s University’s Schiek said border posts could be avoided in two ways: if the UK government were either to exit the customs union but renew the relationship under a different name (similar to the customs union arrangement Turkey has with the European Union, despite not being an EU member state) or to agree that Northern Ireland could leave the European Union with the United Kingdom but remain in the EU customs union.

An EU internal paper leaked to the media cites the second option as the only way to avoid a hard border, so that the same rules would apply throughout the island of Ireland — a position supported by the Irish government.

But this is a political non-starter because of objections by unionist parties in the north.

“The downside from the perspective of the unionists in Northern Ireland would be that Northern Ireland wouldn’t be in a customs union with the UK itself. We would have a customs border in the Irish Sea,” Schiek said.

“That’s very difficult politically because in Northern Ireland there is a large faction who would never want to see any border towards Great Britain, and because the common EU membership of Ireland and the UK is going to end: either there will be a border between Northern Ireland and the Republic of Ireland, or there will be a border between Northern Ireland and Great Britain. There is no alternative. If this could be discussed rationally then one could probably feel that the border in the Irish Sea is less complicated than the land border.”

Barrett believes there is no chance that the customs border will be in the Irish Sea after Brexit, partly because May’s Conservative government is dependent on the support of Northern Ireland’s Democratic Unionist Party, which is strongly opposed to such a move. “Economically, it doesn’t make any sense anyway,” Barrett said. “About two-thirds of Northern Ireland trade is actually with mainland Britain. So, inconvenient though it is, having controls at the border with the Republic actually makes more sense economically.”

In a recently released book from Oxford University Press, The Law & Politics of Brexit (edited by Federico Fabbrini), professors John Doyle and Eileen Connolly of Dublin City University suggest that Northern Cyprus could be a model for Northern Ireland. Goods produced in Northern Cyprus — a territory currently occupied by Turkey, which is not an EU member state — are allowed to enter EU markets without customs duties, once they have been certified by the Turkish Cypriot Chamber of Commerce.

Doyle and Connolly write: “A similar arrangement could empower the Northern Ireland executive to identify goods as originating in Northern Ireland (and not simply travelling through Northern Ireland from the UK or a third country). As part of a Brexit deal, the EU could allow certified goods from Northern Ireland to enter the EU market via the Republic of Ireland and to be treated as EU goods. At the same time, the UK could allow such goods [to] enter the UK market as ‘domestic goods’, and this would allow the UK to present this arrangement as a symmetrical one, meeting the needs of both nationalists and unionists, and therefore acceptable to the unionist parties and to pro-Ulster Unionist conservatives.”

However, many have argued that in the event Northern Ireland was granted unique status in terms of trade, Scotland would be encouraged to seek a similar arrangement (the overall UK vote on Brexit was 52 percent for, 48 percent against, whereas in Northern Ireland, the result was 44 percent for and 56 percent against. In Scotland, the result was 38 percent for and 62 percent against).

Schiek believes there is no basis for this: “There is a decisive difference. There is no such thing as a Good Friday Agreement between Scotland and the UK, and rightly so, because the GFA takes into account that Northern Ireland is a hybrid territory where there are people who identify as Irish, people who identify as British, people who identify as both and people who identify as neither, and the GFA guarantees that all these identities can flourish.”

Another border-related issue that has arisen in the context of Brexit is the Common Travel Area (CTA), which predates membership of the European community by either state and enables UK and Irish citizens to travel and reside in either jurisdiction without hindrance, as well as enjoy associated rights and entitlements. The CTA is recognized in EU law by Protocol No. 20 to the Treaty on European Union and Treaty on the Functioning of the European Union.

Concerns have been expressed that the right of Irish and UK citizens to move freely between either jurisdiction might be affected by Brexit, but Barrett thinks this is unlikely: “I’m not expecting that the CTA will either be a bone of contention or that it will be in any particular danger from the process of Brexit. There’s a question mark over whether Protocol 20 will be automatically revoked or automatically cease to apply by virtue of the process of Brexit. You can argue that either way. But even if that were true, there are other provisions and other protocols that would maintain Ireland’s position anyway. So it’s not in any danger.”

Schiek is less optimistic and believes the position needs to be nailed down: “Presently the CTA is not a proper international agreement between Ireland and the UK. It is just common practice through usage which has been agreed between the two governments but is not legally binding. So if one state is outside the EU and the other is inside the EU, it would be very good if they could agree a legally binding document.”