Modern history is punctuated with significant dates marking the changed relationship between Ireland and Britain. The Easter Rising of 1916; the advent of the civil rights campaigns in 1968; the hunger strikes of 1981; and the signing of the Good Friday agreement in 1998. We can now add to those dates the Brexit referendum of 2016. In the words of WB Yeats, all has changed, and changed utterly.

This vote has placed the progress made since 1998 in jeopardy. The vote of the people of the north of Ireland to remain in the European Union has been ignored. The Irish government, as joint co-equal guarantor of the agreement, has been sidelined.

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The people of Ireland observe the breathtaking arrogance of leading Conservatives who repeat the Thatcherite ignorance that the north of Ireland is as British as Finchley (Margaret Thatcher’s former constituency), or Britain’s border in Ireland is the same as the boundary between London boroughs. The jingoism of the debate about Brexit has never featured Ireland, our agreements, our people or our economy.

Opposition to Brexit united many in the north, and a majority voted to remain. Parties across the political spectrum recognised the damage of Brexit and campaigned against it. The only Brexit supporters were the Democratic Unionists – who opposed marriage equality, opposed language rights and the funding of inquests to victims of the conflict. They’re also a party whose actions in the renewable heat incentive financial scandal collapsed the northern executive last year.

The Good Friday agreement obliges the British government to act with “rigorous impartiality”. Yet following last year’s UK general election, the British prime minister partnered with the DUP in a confidence and supply deal.

This deal robbed Theresa May and her government of any pretence of impartiality in the negotiations for the restoration of the north’s political institutions. The idea that her government could act as an honest broker while beholden to the DUP stretches credibility to breaking point.

We had reached an agreement with the DUP that would have seen the executive back up and running by now. Sadly, the DUP leadership failed to close the deal and collapsed the talks process. They now demand a return to a form of direct rule that ended decades ago, while the Tories allow the DUP, joined at the hip to the hard-right Brexiteers, to apply a destructive veto on Britain’s Brexit policy.

The DUP is acting against our will and our interests in pursuit of a narrow party-political objective

May says she wants no hard border in Ireland. But while the EU has brought forward proposals in the draft treaty to ensure this, the British government has rejected them outright without proposing viable alternatives.

The British government has said there can be no barriers to trade from the north to Britain. This is completely within its gift, yet it has refused to bring forward a proposal on this matter. If May’s government thought it could advance negotiations on the future relationship with the EU while kicking the can down the road on Ireland, then European council president Donald Tusk’s assertion of “Ireland first” has disabused of that notion.

The DUP is acting against the democratic will of the people and against our economic interests, in pursuit of a narrow party-political objective. They are for, as one DUP spokesperson said, “Brexit at any cost”, without any special arrangements for the north. This position is profoundly undemocratic. The Good Friday agreement belongs only to the people of Ireland north and south – it is not a Tory Brexiteer bargaining chip.

Progress made since 1998 can only be maintained when the Tories recognise that the north is a special case. The Dáil, the Irish government and the EU commission all support special circumstances for the north, which include remaining within the customs union and the single market. Moreover, the EU has made the avoidance of a hard border and safeguarding the entire Good Friday agreement key objectives in Brexit negotiations. Only today, a UK parliamentary committee said any possibility of a “frictionless” border is years away, given the lack of progress on technology.

Incredibly a return to direct rule, the shredding of the Good Friday agreement and a hard border in Ireland are being talked about by Tories as legitimate and acceptable outcomes. That is utter madness. Those outcomes would signal a complete political breakdown between Ireland and Britain.

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Governments come and go. The DUP-Tory pact will eventually unravel but its legacy will remain: further division in the north, gerrymandered electoral boundaries, economic regression and a hard Brexit border in Ireland.

Or the British government could do the right thing, stand by the Good Friday agreement, respect the vote in the north and accept that special status for the north with the EU is the way forward.

But regardless of how the British government chooses to act, to the people of Ireland at least one thing is clear: the planned imposition of Brexit in Ireland has again demonstrated the failure of partition, and exposed further the gaping democratic deficit inherent in a partitioned Ireland. In this vein, Brexit has cast the project for Irish unity as an urgent, desirable and realisable political objective. All has changed, changed utterly.

• Mary Lou McDonald TD is leader of Sinn Féin