Unionism in Northern Ireland is dying out – just look at the fall of the house of Paisley | Emma DeSouza

After 14 years of Conservative dominance, Britain has ushered in a new era of politics: record-breaking losses for the Conservative party, Keir Starmer’s whooping victory, the wipeout of the Scottish National party (SNP), growth for the Liberal Democrats and the return of Nigel Farage. But across the water lies another electoral earthquake that has utterly rearranged the political landscape of Northern Ireland: the fall of the house of Paisley.

With only 18 Westminster seats, Northern Ireland was low on the political agenda during the snap election – that is, until news began swirling of a shock loss in North Antrim for the Democratic Unionist party (DUP). North Antrim was considered the party’s safest seat. This was not only a unionist heartland but also Paisley-land. The seat has been held by a member of the Paisley family for 54 years – won by DUP founder Rev Ian Paisley in 1970 and held by his son Ian Paisley Jr since 2010. The loss of North Antrim by 450 votes ended the Paisley dynasty and has raised questions about the future not only of the DUP but also of unionism itself.

The losses did not end there. While DUP leader Gavin Robinson held his seat in East Belfast, the party suffered additional losses in Lagan Valley and South Antrim. The largest unionist party in Northern Ireland has returned to its lowest number of seats in Westminster since 2001. Robinson, who sits on the moderate wing of the party, increased his vote share, while those further to the right were punished. This suggests voters may want a more progressive voice.

In 1921 Northern Ireland was established with an in-built unionist majority intended to maintain Northern Ireland’s place within the United Kingdom and unionism’s dominance within Northern Ireland. Fast forward a century, and unionism has lost its majority at every level of public office. This isn’t merely a bad election or a temporary decline; unionism is facing a rapid existential crisis. The DUP’s vote share was consumed at both ends of the political spectrum; the Alliance party, which describes itself as neither unionist nor nationalist, took Lagan Valley – a seat held by former DUP leader Jeffrey Donaldson for 27 years. South Antrim, meanwhile, was snatched by the more moderate wing of unionism, the Ulster Unionist party (UUP), while North Antrim was won by the more hardline Traditional Unionist Voice (TUV).

Unionism as an ideology is broken, fracturing itself to pieces on an ever-shrinking board. The 2021 census illustrated a sharp decline in British identity in Northern Ireland, and survey results consistently indicate that a growing number of people – particularly those aged 18 to 24 – feel no affinity towards unionism. With a decreasing voter base, it is difficult to imagine three parties representing three wings of unionism surviving the next decade and, as this political ideology slips further towards oblivion, so too will support for staying in the United Kingdom.

Though the causes of unionism’s decline are multifaceted, this has undeniably been hastened by Brexit. The DUP’s folly in supporting the UK’s departure from the European Union, and the subsequent propping up of Theresa May’s Conservative government, galvanised pro-European and nationalist voters in Northern Ireland while simultaneously causing an irreparable rift within the unionist cause.

Irish nationalist party Sinn Féin, which advocates for a united Ireland, has become the largest Northern Ireland party across the board at the devolved assembly, local council and now Westminster. Sinn Féin’s historic hat-trick is the clearest indicator of Northern Ireland’s shifting demographics and political aspirations. As the party that helped deliver the Belfast/Good Friday agreement, the incoming Labour government needs to discharge its duties under the agreement by outlining the criteria for calling a border poll.

The power to call a referendum on Ireland’s constitutional future lies entirely in the hands of the secretary of state for Northern Ireland, but the criterion for doing so remains vague. The Good Friday agreement places a duty on the secretary of state to call a border poll “if at any time it appears likely to him that a majority of those voting would express a wish that Northern Ireland should cease to be part of the United Kingdom and form part of a united Ireland”. How exactly will Hilary Benn, the newly appointed secretary of state for Northern Ireland, assess the constitutional aspirations of the electorate of Northern Ireland? It is unhelpful that the criteria by which such a seismic decision will be made remain cloaked in secrecy.

The nationalist party, which abstains from taking its seats at Westminster, not only held its seven Westminster seats but significantly increased its vote share. Fermanagh and South Tyrone was the most marginal seat in the United Kingdom – not any more: Sinn Féin’s Pat Cullen increased the party’s majority to 4,571. Elsewhere, the party depleted the majority of Social Democratic and Labour party (SDLP) leader Colum Eastwood from 17,110 down by more than three-quarters to 4,166 and very nearly unseated DUP grandaddy Gregory Campbell in East Londonderry , whose majority of more than 9,000 was reduced to just 179 votes by Sinn Féin’s Kathleen McGurk. Unlike the DUP, Sinn Féin has clear electoral targets for further growth at the next election.

Related: The Guardian view on the general election in Northern Ireland: time for London to re-engage | Editorial

Before then, Starmer will be faced with the most diverse representation from Northern Ireland on the green benches in decades. The days of DUP dominance are over. Labour’s focus will undoubtedly be on England, but should it overlook the changing landscape in Northern Ireland, Labour could well find itself sleepwalking into another ill-planned referendum with profound effects on relationships across these islands. Rather than ignore reality, the British government should begin working with the Irish government to lay out a path towards a vote on Northern Ireland’s place in the United Kingdom.

Preparing the people of Northern Ireland for a border poll will require considerable time and resources. If we want to avoid another version of Brexit, a detailed plan as to what a united Ireland would look like, including clarity over a potential transition period, would need to be developed. It has been said that no prime minister wants to preside over the breakup of the United Kingdom. But Labour should embrace the preparations for a referendum as a means of honouring the principles of the Good Friday agreement. Overseeing an amicable democratic vote on self-determination as enshrined in the historic peace agreement should be revered as a privilege, one that any courageous leader would hope to undertake in their career.

• Emma DeSouza is a writer, campaigner and peacebuilder