Among the E.U.'s core principles is the free movement of goods and labor, which normally would automatically lapse once a member state, such as the United Kingdom, leaves the union. The Dublin-based Irish government had threatened to veto any Brexit deal that would result in a hard border between the Republic of Ireland and Northern Ireland, which belongs to the United Kingdom, however.
To Britain, Ireland's threatened veto may have been the reflex of a smaller nation punching far above its weight.
But to Ireland, it was a matter of preserving peace.
The two countries are currently not separated by any visible border — which is quite common within the European Union — and there were fears that a reintroduction of checkpoints could threaten the peace process on the island, which remains volatile. There are still occasional attacks by Northern Irish separatists in the region, even though the violence mostly faded away following the 1998 Good Friday Agreement.
Whereas Britain is still set to leave the European Union's single market, May has now promised that Northern Ireland will maintain “full alignment” with E.U. customs and trade regulations and be able to uphold its borderless trade with European Union member Ireland. The deal will please the government in Dublin and most voters in Northern Ireland, who predominantly favored staying in the European Union in last year's referendum.
For May, the arrangement has multiple downsides. It may trigger similar demands in other pro-E.U. parts of Britain — such as Scotland and London — to also maintain closer ties with Europe. Moreover, it disgruntles the tiny and yet crucial Northern Ireland Democratic Unionist Party (DUP), on which May's Conservatives rely to sustain their government. (May lost the overall majority in elections this summer). The DUP is a strong advocate of close ties to the United Kingdom and remains opposed to closer relations with the neighboring Republic of Ireland.
The Irish ties, however, have helped to overcome a decades-long divide and are needed to sustain peace, pro-E.U. campaigners argue.
The origins of that dispute go back to the early 1920s when an Irish revolt against British rule gained momentum. As a result, the island was split in half: into a separate, independent country in the south and into less autonomous Northern Ireland, which has remained part of the United Kingdom ever since.
The split-up led to a violent revolt among separatists in the north, who soon deployed terrorist tactics there that to some extent continue until today. Although the number of mass-casualty attacks remained small, Europe became much more used to frequent attacks at the time — mainly carried out by the Provisional Irish Republican Army (IRA) and its splinter groups — than it is today.
The divide also went along sectarian lines, and it still does. Whereas Catholics constituted the majority of the population on the island overall, they were in the minority in Northern Ireland. Mostly in favor of a united, independent Ireland, Catholics there faced repression by the mostly Protestant so-called unionists, who are today represented by the DUP.
Violence between the two groups escalated in the 1960s and resulted in a major deployment of British soldiers to the region. Deployed to protect Catholics there, British troops soon became targets of an insurgency that played out both in Northern Ireland and the rest of Britain.
Even today, a wall separates Catholics and Protestants in a part of Belfast, the capital of Northern Ireland where the tensions ran highest.
Thousands were killed before the 1998 Good Friday Agreement was eventually signed to bring peace to the region. A new parliamentary assembly, composed of representatives of all communities, was supposed to stabilize the volatile part of the United Kingdom. The peace mostly held, even though there have been concerns over a rise in militant activity in recent years.
Public support for militant groups has mostly disappeared, however, as many Northern Irish have come to appreciate the benefits of peace and cooperation with Ireland. But the crucial agreement on which the peace is based includes more than 100 references to E.U. regulations and rules, and renegotiating the deal could reopen old wounds, some fear.
"Peace can be thrown out of the window if we reintroduce British checkpoints across the border," Britain's former European Union minister Denis MacShane told a French media outlet prior to Friday's agreement.
The weeks-long struggle to find a compromise, with the Irish side ultimately winning, indicates that the risk was indeed too real to simply be ignored by the British government.