When Irish republicans sat down with members of the British government in 1921 to negotiate what became the Anglo-Irish Treaty, ending Ireland’s war of independence, the state of Northern Ireland, and thus the partition of Ireland, was already a reality, having formally come into being earlier that year.

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To prevent the negotiations breaking down over Irish republican demands for an end to this partition, the British government concocted a cunning plan for a boundary commission to review the border in accordance with the “wishes of the inhabitants” and with due regard to economic and geographic conditions.

Irish negotiators maintained the commission would produce favourable findings that would lead to a significant territorial gain, ultimately making unity with Northern Ireland much more likely. The eventual report of the boundary commission in 1925 – involving only a minimal transfer of territory from Northern Ireland to the south, and even some transfer in the other direction – shattered these illusions, leading to a hasty agreement between Dublin, London and Belfast to leave the border as it was. And so it has remained since, all 300 miles of it.

Historical context is essential to understanding the significance of the news that Britain wants to shift its immigration controls back to Irish ports and airports to avoid the introduction of a “hard” border between Northern Ireland and the Republic after Brexit. Despite much rhetorical and emotional attachment to reunification, and a disastrous IRA campaign targeting the border from 1956-62, pragmatism has dominated the southern Irish approach to the border question. Anglo-Irish and north-south relations noticeably improved from the 1960s, with a particular focus on trade. The outbreak of the Troubles, from the late 1960s, led to numerous crises but was eventually solved by the peace process and the Good Friday agreement of 1998. This agreement maintains that it is “for the people of the island of Ireland alone, by agreement between the two parts respectively and without external impediment, to exercise their right of self-determination”.

Facebook Twitter Pinterest Passport control at Dublin airport, where Britain wants tighter controls to be imposed. Photograph: Niall Carson/PA

Following the Brexit vote in June, when 56% of Northern Ireland’s electorate voted to remain in the EU, historian Ian McBride argued that the Good Friday agreement “clearly envisaged that Northern Ireland’s future constitutional arrangements would be worked out in the context of continuing partnership between the north and the south, and between politicians in London and Dublin”. And he added: “To remove Northern Ireland from Europe without its consent is not only morally wrong and politically risky; it is also a rejection of the fundamental bilateralism of the peace process”.

The Brexit result also raised this bizarre question: given that, under the Good Friday agreement, those born in Northern Ireland can be citizens of the UK or Ireland or both, can they now be both EU and non-EU citizens?

During the referendum campaign, the then Northern Ireland secretary, Theresa Villiers, claimed that the Common Travel Area between Britain and Ireland – based on administrative agreements of 1922 and 1952 confirming the special status of Irish citizens in Britain and vice versa – means there is no need for a “hard border”. Now her successor, James Brokenshire, insists London and Dublin will work closely together to “strengthen the external border” of the travel area in order to combat illegal migration into Britain once it leaves the EU.

One hundred years after the 1916 Rising, older arguments about Irish psychological independence may resurface

But the reality is that the Common Travel Area would be an external border to the EU as a whole; and what Britain may regard as a better alternative to a “hard” Irish border may be wishful thinking. For all the soothing rhetoric from Irish foreign affairs minister Charles Flanagan about sharing information and systems to counteract illegal immigration, he also warns: “This will be a decision not just by the UK or Irish governments but ultimately also by the 27 EU states.”

This will be a difficult balancing act, given Ireland’s continuing membership of the EU. Brexit represents the Republic’s greatest challenge since joining the EEC in 1973, because the Irish government needs to emphasise both Irish distinctiveness from Britain and the two nations’ common needs. Senior members of the Irish government, because of their fear of a “hard” north-south border, are playing down the significance of Brokenshire’s suggestions; but moves to make ports of entry proxy border posts for Britain are likely to stir opposition. One hundred years after the 1916 Rising, some of the older arguments about Irish psychological independence may resurface as a reaction to Britain seeking to “use” Ireland and its borders to assert its new isolationist status.

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Almost 40 years ago, a senior Irish civil servant, irritated that the Republic was being taken for granted by Britain, urged the Irish government “to bring out once more again the fact that we are not an appendage of the British in the European communities”.

In the last few years such tensions have eased, and there have been frequent and accurate assertions that Anglo-Irish relations have never been better. The sense of an “invisible” Irish border has also greatly improved relations between Northern and southern Ireland. The “soft border”, over which 30,000 people travel each day, is even more valuable now as both economies are exposed to the consequences of Brexit.

When he took over as Ireland’s first minister for external affairs in 1922, Desmond FitzGerald said simply: “Britain is our most important external affair.” That was because of what Britain and Ireland had in common, but also because of what fundamentally divided them. This Irish border question has the alarming potential to undermine much of the painstaking progress that has been made in Anglo-Irish relations, shifting the emphasis away from reconciliation and common interests, and back to what divides.