UK 15 May 2018 Is Theresa May right to worry about a hard border causing a united Ireland? The Prime Minister’s row with Jacob Rees-Mogg over what would happen in a post-Brexit unification referendum raises existential questions. Getty Images Sign UpGet the New Statesman's Morning Call email. Sign-up Is a united Ireland inevitable in the event of a hard border? Some think so, and Theresa May, it seems, isn't a million miles away from that conclusion either. Today's Times reports an exchange between Jacob Rees-Mogg and the Prime Minister on the prospect of a referendum on Irish unification after Brexit during a No 10 briefing for MPs yesterday. In comments not disputed by Downing Street, May reportedly told the European Research Group chairman that she was not confident that Northern Ireland would vote to remain in the UK should border infrastructure be built after Brexit. The remarks are worth considering in full. A source told The Times: "Jacob said: 'If there was a border poll, I have no doubt we would win, as the UK did in Scotland.' "Mrs May said, 'I would not be confident as you. That's not a risk I'm prepared to take. We cannot be confident on the politics of that situation, on how it plays out.'" There are several questions to consider here. The first, and least interesting, is whether the Scottish independence referendum is a reasonable point of comparison for Unionists ahead of a potential border poll in Northern Ireland. The answer to that is a straightforward no. As is too often forgotten in the Brexit debate, Scotland is not Northern Ireland and Northern Ireland is not Scotland. The origin story of any border poll would naturally be much more fraught, its conduct even more acrimonious, and the electoral dynamics entirely unique. The terms of the Good Friday Agreement – which says that any decision over the future of Northern Ireland is for the people to take without external impediment – would almost certainly constrain the role of the British government, ruling out the prospect of an Ulster version of the Unionist side's "Project Fear". And the result of the 2014 Scottish referendum proved anything but conclusive. A relatively narrow win for Unionism, comparable to that vote, would not resolve the question for a generation, but would likely encourage a one-more-heave mentality among nationalists. The constitutional question would remain electrified and recreating a functional, shared polity in Northern Ireland against this backdrop would prove even more difficult. The exchange also raises the issue of who is right - is it the confident Rees-Mogg or the anxious May? Much of the polling one might use to consider that question is an imperfect fit as it does not take into account the possibility of border infrastructure. Though Brexit does not equal a united Ireland, there is some evidence that May would be right to worry should the worst case scenario be eventually realised. A survey by LucidTalk, Northern Ireland's sole pollsters, found in December 2017 that 47.9 per cent of Northern Irish voters would vote for unification if Brexit meant "leaving the EU with no deal on the border, the Good Friday Agreement or citizens' rights", while 45.4 per cent would vote to remain in the UK (with 6 per cent undecided). That, of course, isn't quite the same as the scenario imagined by May – where a unilateral British pledge to maintain an open border despite having left the single market and customs union, as suggested by Rees-Mogg, would result in Ireland and the EU building infrastructure on the frontier, with no mention of agreement on other issues (it's worth noting that the poll was commissioned by Sinn Fein's group in the European Parliament). The significant gap between the Brexit-specific survey and others (support for a united Ireland typically hovers around a third), nonetheless shows how a hard Brexit could fundamentally alter the dynamics of the unification question. More specifically, May would be right, as the Times suggests she was, to worry about upsetting the substantial minority of nationalist voters who would not necessarily back a united Ireland in a border poll were one held tomorrow (and, indeed, alienating liberal Unionists). Downing Street is also conscious of more general polling that suggests rising support for Irish unity. From this follows another important point: why talk about the prospect of a border poll at all? Privately, some Unionists complain May's comments implied that the criteria for such a vote had been fulfilled. The Northern Ireland Secretary can theoretically hold a border poll at any time, but the Good Friday Agreement states they must if they believe a majority will vote in favour of a United Ireland. That isn't quite what May was reported as saying – and Karen Bradley has since said she does not believe the criteria have been met – but it hasn't stopped Sinn Fein from interpreting it as such. There, arguably, lies the problem with using the question of Northern Ireland's future to cow Brexiteers: doing so can also mean giving succour to the nationalist cause and seemingly prevaricating over the strength of the Union. Neither will please the DUP or ERG, but the government must also find some way to meet its pledges on not imposing new border infrastructure. Rows like this illustrate that it will be near-impossible to achieve both. › After 22 years I’ve stopped taking antidepressants – we need a more nuanced debate about them Patrick Maguire was political correspondent at the New Statesman. Subscribe For daily analysis & more political coverage from Westminster and beyond subscribe for just £1 per month!