Brexit will have many impacts. But perhaps one of the most significant ones concerns the only physical frontier between the UK and the EU – the 499 km border between Northern Ireland and the Republic of Ireland. This is set to become the EU's external border once the UK leaves the EU.  

On 9 April, the twentieth anniversary of the Good Friday Agreement, former US Secretary of State Hillary Clinton repurposed Tony Blair's comments of 1998 and said "If short-term interests take precedent over solving the long-term challenges that still exist in Northern Ireland, then it is clear that the hand of history will be both heavy and unforgiving."

Why is this the case?

First and perhaps most paramount: the Good Friday Agreement saw the permanent dismantling of border controls and is considered as representing the end of "the Troubles" – a series of sectarian violence which plagued Ireland and Northern Ireland in the second half of the 20th century. The UK and Irish governments negotiated the Good Friday agreement, with the EU playing an integral facilitator role and remaining co-guarantor to this day. Given the EU was initially created to ensure peace and the UK/Irish governments brokered the agreement, it is understandable that all sides want to avoid a return to a physical border.

Secondly, from the British government's perspective, the constitutional integrity of the United Kingdom of Great Britain and Northern Ireland is symbolically and politically vital. After all, the full, official name of the Tory party remains the "Conservative and Unionist party". Theresa May's government is formally linked to the Northern Irish DUP party via a confidence and supply agreement.  These are the primary reasons why the UK government reacted so angrily at the EU's suggestion of considering a separate and distinct regulatory regime post Brexit for Northern Ireland from the rest of the UK. However, seen from Dublin (with full support of Brussels and all EU 27 capitals), the UK's insistence on leaving the EU's customs union and single market means the unacceptable potential for two sets of diverging rules on the island that includes Ireland and Northern Ireland. In order to avoid a hard border but avert dual regulatory regime, the UK has committed to a "fallback solution" of regulatory alignment, whilst insisting a more innovative, technological solution to ensuring a frictionless border is possible. Finding a mutually acceptable solution is perhaps the single largest outstanding "divorce" issue to be dealt with in the coming weeks and months.

Finally the economic and regulatory factor: Brexit will inevitably lead to a different economic and regulatory relationship between the UK and EU. Realistically this will have to be less integrated than currently is the case. Quite how much the economies on either side of the Ireland and Northern Ireland border are impacted will depend on the final trade relationship. The closer the relationship, the lesser the impact. Conversely, the looser the relationship, the greater the impact. What is critical from an EU perspective is to avoid circumvention of the EU's regulatory regime, creating a back door into the single market for third country goods from across the world. This is why the EU is so insistent that a frictionless border is not compatible with leaving the customs union/single market.

When approving Mr Barnier's political guidelines recently, EU leaders committed to looking again at where things stand on the Ireland question at its next meeting in June. With less than a year to go until the UK formally leaves the EU, the political focus remains on amicably concluding the divorce before the real discussions on starting afresh can begin. Paradoxically Brexit has put one of the EU's smaller member states front and centre – or as European Council President Donald Tusk put it: Ireland first. Proof enough of why the Ireland and Northern Ireland question is so important.

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