There is no doubt that the supporters of Brexit, avowed unionists all, have done more to advance the cause of a united Ireland than the IRA ever managed in 30 years of terrible violence. The simplest way to avoid a hard border on the island of Ireland, after all, is to have no border at all.

Northern Ireland voted against Brexit and will be very deeply and adversely affected by it. And the European Union has quite explicitly – and quite remarkably – stated that Northern Ireland can automatically rejoin the EU at any time after Brexit takes effect if it agrees to a united Ireland. In this, Northern Ireland is already different from Britain: its exit from the EU is a return ticket.

Quick guide Why is the Irish border a stumbling block for Brexit? Show Hide Counties and customs Inside the EU, both Ireland and Northern Ireland are part of the single market and customs union so share the same regulations and standards, allowing a soft or invisible border between the two.

Britain’s exit from the EU – taking Northern Ireland with it – risks a return to a hard or policed border. The only way to avoid this post-Brexit is for regulations on both sides to remain more or less the same in key areas including food, animal welfare, medicines and product safety. The 'backstop' in Theresa May's Withdrawal Agreement was intended to address this - stating that if no future trade agreement could be reached between the EU and the UK, then rules and regulations would stay as they are. This has been rejected by Brexit supporters as a 'trap' to keep the UK in the EU's customs union, which would prevent the UK striking its own independent trade deals. There are an estimated 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, and about 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. Brexit supporters say this can be managed by doing checks on goods away from the border, but critics say it will be difficult to police this without any physical infrastructure like border posts or cameras, which could raise tensions in the divided communities of Ireland.

Interactive: A typical hour in the life of the Irish border Photograph: Design Pics Inc/Design Pics RF

Meanwhile, Brexit and the upsurge of English nationalism that drives it have created a deep existential crisis for the United Kingdom that will unfold over the next 20 years. The crisis of authority that is already so evident in London is likely to become even more acute: any conceivable deal with the EU will reignite the uncivil wars in English politics. Scotland’s resentment will deepen if an economic crisis results from a revolutionary change it emphatically rejected. With such anarchy in the UK, any sensible unionist should be looking across the Irish Sea and asking – what the hell is it we are supposed to be united to? There is no British status quo to maintain.

For any thinking unionist, it has long been clear that the key to keeping Northern Ireland in the union is Catholic consent. As demography erodes the inbuilt Protestant majority, the sectarian headcount is no longer enough. Enough Catholics in the north have to be persuaded of the benefits of the current arrangements not to want to risk the great uncertainties of change.

And the odd thing is that before June 2016 this was, from a unionist perspective, going rather well. The equality guaranteed in the Good Friday agreement, alongside the in effect borderless existence created by common membership of the EU, meant that, whatever their long-term aspirations, most Catholics were willing to live with things as they were.

It is quite breathtaking that the Democratic Unionist party in particular has failed to grasp the simple fact that this Catholic consent was their greatest asset and Brexit throws it away. The polling evidence is stark. Only 28% of Catholics in Northern Ireland would vote for a united Ireland if the UK changed its mind and remained a full member of the EU. However, 53% of Catholics would vote for a united Ireland if there were a hard Brexit in which all of the UK left the customs union and single market. This figure is likely to increase as the consequences accumulate in the coming years.

Ireland has come too far to be dragged back in time by Brexit | Paul McGrade Read more

But just because unionism is apparently intent on suicide, we should not be quick to dance on its grave. The great recklessness of Brexit from an Irish point of view is that it forces people to think again about all of the big existential questions that have caused so much grief and that had been successfully suspended. Whatever one thinks about a united Ireland, it is simply too much too soon. The kind of unity Ireland needs is not about territory – it’s about people. The Irish constitution was amended 20 years ago to redefine the national aim as being “to unite all the people who share the territory of the island of Ireland, in all the diversity of their identities and traditions”. We are not remotely there yet. We have a slow, delicate process of healing and reconciliation that Brexit tramples on with oafish disregard. We need time. And you don’t have to be a rabid nationalist to think time to heal is the least Britain owes Ireland.

Fintan O’Toole is a columnist with the Irish Times