Ireland’s remarkable general election clearly marks a turning point for the republic. By topping the poll last weekend, Sinn Féin has broken the old politics wide open. Mary Lou McDonald’s party will now be either part of Ireland’s new government or the main opposition. Either outcome marks an astonishing change of fortune. Only nine months ago, in last year’s European elections, Sinn Féin came a poor fifth in seats. Within living memory, the party was a virtual pariah in the south.

The twin questions of how this happened and what it means for the future are now subjects of intense debate. Two points stand out so far. First, that Sinn Féin overwhelmingly owed its success to domestic left-of-centre pitches on subjects such as housing, health and pensions, not to its demands for Irish unity. And second, in Fintan O’Toole’s words, the heart of the result is Ireland’s desire for something that looks a lot like traditional European political normality. The voters, he argues, have decreed that the two centre-right parties that have carved up Irish politics since the civil war have had their day – and that Ireland will be better off ending Sinn Féin’s half-in/half-out status.

Yet Ireland’s election also marks a turning point for the United Kingdom. This may seem a perverse claim, having just said that the Irish election was mainly about affordable housing, hospital beds and a wish to leave the political past behind. But two different things can be true at the same time. Sinn Féin came top of the poll because of its economic and social policies, but it indisputably also stands for the creation of a united Ireland. Its success will inevitably make the pressure for an Irish unification poll – to unite the republic and Northern Ireland – significantly greater than it already was.

The UK left the EU two weeks ago, but in Northern Ireland the withdrawal agreement remains unfinished business

The unification issue became dormant after the Belfast/Good Friday agreement ended the Troubles in 1998. The agreement acknowledged Northern Ireland as part of the UK; but it provides for future unification if majorities in both parts of Ireland agree. The 2016 Brexit vote reawakened the possibility. It threatened to create a new land border between the UK and the EU in Ireland. Boris Johnson’s withdrawal agreement with the EU then removed that. Instead, it placed much of Northern Ireland’s economy under EU rules, with a border now planned in the Irish Sea.

The UK left the EU two weeks ago, but in Northern Ireland the withdrawal agreement remains unfinished business. The deal gives Northern Ireland no say in the EU rules that govern its economy. Last month, the restored Stormont assembly refused consent to it. Northern Ireland’s politicians want a say in the UK’s future relationship talks with the EU, yet they have less leverage to achieve that aim than in the last parliament. And if Northern Ireland gets a say in the talks, how can Scotland or Wales be denied one? It’s not going to happen.

At the same time, it has become increasingly clear that the Johnson government expects to strike a minimalist trade deal with the EU. That would not work for Northern Ireland or the republic, large parts of whose economies operate on an all-Ireland basis. It is therefore Brexit, not Sinn Féin, that has made full political union between Northern Ireland and the Irish republic (and thus the EU) a much more viable option than it was in the past.

If Brexit evolves on Johnson’s terms, Northern Ireland will become a constitutional adjunct of the UK, with its economic terms set in Brussels and shaped by Dublin. That may not be enough to flip Northern Ireland’s divided public straight into the arms of the republic. But it is likely to wear away at the issue over time. The most recent poll, by Lord Ashcroft last September, showed narrow overall support in the north for unification. That lead may have widened since because of events in both the UK and Ireland.

In the republic’s election exit poll last weekend, 57% backed new unification referendums in the two jurisdictions. That may seem quite modest and cautious. But if Sinn Féin is in government, it is likely to set in motion a process towards an eventual unification vote. If it is in opposition, it will pressure a necessarily weak government into doing the same. One way or another, a unification poll is now likely to receive more support in Dublin than it has in recent decades. That is an important change. The former taoiseach Bertie Ahern, the ultimate pragmatist, is surely right to suggest that a vote is now inevitable in about five years or more.

The unification of Ireland would not be like the reunification of Germany 30 years ago. In Germany, the communist eastern state collapsed and most Germans were keen to embrace one another. In Ireland, it would be different and messier. The terms of any referendum would be crucial, the size of the majority even more so. Suppose, as Jonathan Powell, one of the architects of the Belfast agreement, recently suggested, that the margin for Irish unification in Northern Ireland was similar to the Brexit vote – 52% to 48%. Both the republic and the UK would be bound by the 1998 agreement to implement it. Will Johnson force the 48% into the republic? The likelihood of violence from either loyalists or dissident republicans would be very high, depending on the course he chose. And if Johnson agrees to a border poll in Ireland, how can he continue to refuse one to Scotland?

These are now extremely serious possibilities for the United Kingdom. They will be exceptionally difficult to resist, even under a London government that, unlike Johnson’s, had the skills, judgment and goodwill to attempt the task. Clear warnings were made, both in 2016 and since. They should have been heeded. Instead they were ignored. In the end, it may be democratically impossible to prevent the breakup of the UK. But let no one forget where the responsibility for that will lie. It will be a direct and foreseen consequence of Brexit. The success of Sinn Féin merely makes it a bit more likely than before.

• Martin Kettle is a Guardian columnist