In the run-up to next month’s critical EU summit, when the 27 heads of government will decide whether to move to talks on the future relationship between the EU and Britain, Ireland has assumed centre stage. This should come as no surprise to anyone in the UK, as successive Irish governments have been telling them since David Cameron first mooted a Brexit referendum in 2013. Brexit threatens Ireland’s core geopolitical and geoeconomic interests.

The taoiseach, Leo Varadkar, has in recent weeks become much more assertive in his insistence that the UK government addresses the border problem before trade talks can begin. This has earned him the ire of the British tabloid press and of the DUP leader, Arlene Foster, whose party holds the balance of power in Westminster but who warned Varadkar not to “play around with” Northern Ireland. The leak of an internal Irish government document in which the British approach to Brexit was described as “chaotic” won’t have reduced the tensions.

The shock of Brexit for Ireland is explained partly because the time between the normalisation of British-Irish political relations and the decision of the UK to leave the EU has been so short. The so-called Irish question bedevilled British politics in the 19th and 20th centuries. The search for a long-term settlement of the difficulties between our neighbouring islands proved elusive throughout.

The Good Friday agreement, signed in April 1998 and ratified by majorities in both parts of the island, represented a normalisation of relations that was powerfully symbolised by the Queen’s very welcome visit to Ireland in 2011, the first by a reigning British monarch since the foundation of the Irish state.

However, the conflict between the two communities in Northern Ireland was not resolved by the agreement. Rather, it morphed into a nonviolent conflict, an uneasy peace rather than deep reconciliation.

Yet the fact that the long and rambling Irish border is now as invisible as the boundary between Belgium and the Netherlands (albeit with many more crossings) is to be celebrated. It is partly an achievement of the EU single market, which from 1993 facilitated the free flow of goods, while the Good Friday deal led to the abolition of the security apparatus.

Only those who live along a contested border can really appreciate what it means to enjoy its disappearance. Brexit brings back fear, uncertainty and division. It is reminder to the people of Northern Ireland that their society is deeply divided. While 85% of nationalists voted remain, 60% of unionists voted leave.

During the referendum campaign in the UK, the potential impact on Ireland received scant attention, a fact that was noted across the Irish Sea. Aware of Ireland’s vulnerability, the Dublin government began a campaign to ensure that our partners in the EU and European institutions understood Ireland’s vulnerability. This led to recognition in the EU’s negotiating guidelines for the talks that Ireland was one of three top priorities to be addressed before the EU would move on to negotiations about its future relationship with the UK.

There is a widespread consensus across the EU that it is the UK’s responsibility to address the particular problems caused by Brexit for Ireland. The European commission’s Brexit task force paper clearly states: “The onus to propose solutions which overcome the challenges created on the island of Ireland by the United Kingdom’s withdrawal from the European Union and its decision to leave the customs union and the internal market remains on the United Kingdom.”

The Brexit chief negotiator, Michel Barnier, addressed the two houses of the Irish parliament in May and visited the border region, whereas Theresa May turned down an invitation to do likewise.

The sentiment in Ireland is that the decision to opt for a Brexit model that involves leaving both the single market and customs union undermines the credibility of the UK’s stated commitment to no hard border on the island of Ireland, and to frictionless trade.

When the UK exits the EU, it becomes a “third” country, and its only land border with the EU will be the Irish border. The paper on Ireland prepared by the UK Department for Exiting the EU is not accepted as a serious political or policy response to the challenge of the Irish border.

There are two lines of argument frequently aired in the UK that serve to bolster Irish concerns. The first is that the Irish border is a matter for the second phase of negotiations, when trade will be discussed. While superficially plausible, this argument is bogus. There is no future trade deal available to the UK that will lead to frictionless trade if its leaves the single market and customs union.

Second is the argument of the hard Brexiters and the DUP that the UK does not want a border, so any responsibility for one lies with the EU. This claim is mendacious, and is regarded in Ireland as a transparent ploy to dodge blame.

The Irish government needs language from the UK government that demonstrates that it is not in denial about the border and that it has done some serious thinking and problem-solving. This involves a commitment to regulatory convergence between Northern Ireland and the EU. The UK cannot avoid the trade-offs of Brexit forever, and has a moral responsibility to live up to its role as co-guarantor of the Good Friday agreement.

One unintended consequence of Brexit is that Ireland finds itself for the first time in its history on the stronger side of the table in negotiations with the UK. The Irish government can’t overplay this card, but nor should anyone in the UK underestimate the seriousness with which it will hold to its position.

• Brigid Laffan is director of the global governance programme at the European University Institute, Florence