The last time Downing Street foisted a deal on Northern Ireland’s unionists, the backlash was swift and bitter. Hundreds of thousands poured on to the streets to protest. A mob punched and kicked the secretary of state outside Belfast city hall. They hit him with a union jack-draped flagpole, grappled him into a headlock and chanted, “Traitor, traitor, traitor.”

It was 1985 and Margaret Thatcher had signed the Anglo-Irish agreement giving Dublin a say in Northern Ireland’s affairs. It took burly bodyguards to save her secretary of state, Tom King, from the mob’s wrath. Demonstrations, strikes and civil disobedience raged for months. “We say, ‘Never, never, never, never!’” Ian Paisley, the Democratic Unionist party leader, bellowed to a crowd.

Three decades later Boris Johnson has ignored DUP objections and negotiated a Brexit deal that would create a customs border down the Irish Sea. EU regulations would apply to all goods in Northern Ireland and the DUP would lose its veto on whether the new arrangements come into force.

Many unionists inside and outside the party fear for Northern Ireland’s position in the United Kingdom. Even before the deal was unveiled on Thursday, there were cries of betrayal and warnings about tumult and violence. But widespread public disorder seems unlikely.

Billy Hutchinson is leader of the Progressive Unionist party. Photograph: Paul McErlane/The Guardian

“A number of unionists and loyalists are angry, but the political process needs to take their fears away. It’s not the time to ratchet all this up,” said Billy Hutchinson, a Belfast city councillor with the Progressive Unionist party (PUP), which is aligned with the Ulster Volunteer Force (UVF) paramilitary group. “We can’t afford to go back to the bad old days. We won’t.”

Peter Shirlow, a director at the University of Liverpool’s Institute of Irish Studies and an authority on unionism, said there was little appetite for disruption. “A lorry sitting in Larne having its contents checked won’t drive them back to violence. The narrative is changing. Northern Ireland is not as dysfunctional as you think.”

It’s a bold statement, given that Downing Street has called the DUP’s bluff and Northern Ireland has a power vacuum. The prime minister has gambled that he may yet get Arlene Foster, the DUP leader, onside or else shunt the deal through the House of Commons without support from the party’s 10 MPs.

The DUP trapped itself over the past year by turning technical discussions over customs arrangements into “blood red lines” with lurid imagery: “This is a battle of who blinks first, and we’ve cut off our eyelids.” In recent weeks it compromised on regulatory checks and moderated its language, calling for a “sensible” deal, but it stopped short of a full U-turn.

Mistrust between the DUP and Sinn Féin has stymied efforts to revive power-sharing, which collapsed more than 1,000 days ago, leaving the assembly and executive at Stormont mothballed. Add in Sinn Féin’s push for a referendum on Irish unity and attacks by dissident republicans and it is easy to envisage a loyalist backlash.

“A border down the Irish Sea would not be tolerated,” said Jamie Bryson, a prominent blogger and activist. “It would be an economically united Ireland, which would mutilate the constitutional position of the UK. There would be an organic explosion of protest. Nobody knows where that would go. Once you push that genie out of the bottle, it’s a dangerous situation – a powder-keg situation.”

But there seems little appetite for mass unruly protest. Unionism is divided over Brexit: 47% of pro-union voters voted remain in the 2016 referendum. And there is a collective weariness, a desire to move on.

Andrew Graham, manager of the Hillside pub in Hillsborough. Photograph: Paul McErlane/The Guardian

In Hillsborough, a prosperous, largely Protestant village in County Down where Thatcher signed the Anglo-Irish agreement, some expressed dismay at the prospect of Northern Ireland being treated differently to the rest of the UK.

“If there is a betrayal, there will be protests. To what extent I don’t know,” said one retailer, who declined to be named. “It’s like Animal Farm: some people are better than others.”

Other residents said their overriding desire was to avoid a hard border with the Republic of Ireland. Few thought the union with the UK was imperilled.

“I’ve not heard that much conversation about the political side,” said Andrew Graham, the manager of the Hillside pub. “People are worried about house prices, about transport and farming. For those on the border, it could be make or break.”

Emma Grimley, a resident of Hillsborough, County Down. Photograph: Paul McErlane/The Guardian

Emma Grimley yearned for a Brexit deal. “Otherwise, would the food sector get mushrooms from Monaghan? It’s simple things like that. If the country is prosperous and doing well – that’s where my mindset is.”

A business consultant who declined to be named said a small minority could cause ructions but that others would accept a deal. “They’ll have a bottle of shiraz and a nice meal. Betrayal? It’s ridiculous, inflammatory language. Let’s just get on with life.”

A surge in votes for the Alliance, Green and other non-aligned parties in recent elections showed some unionist voters were abandoning tribal traditions, said Shirlow. “There are two pro-union communities and the one that’s growing is the progressive one. You have to realise that 47% who are pro-union voted to remain in Europe.”

Some opposition to the backstop echoed unionism’s old existential dread, but that was balanced by economic pragmatism, said Paul Corthorn, a reader in modern British history at Queen’s University and a biographer of Enoch Powell, who felt Thatcher betrayed him over the Anglo-Irish agreement. “Now there is a separation of forces, a dispersal of unionist opinion.”

Shoppers interviewed on Belfast’s Shankill Road, where murals commemorate the Queen and loyalist paramilitaries, said they disapproved of a border down the Irish Sea, but did not plan to mount any barricades.

“If a deal is done, protests won’t change that,” said Colin Brown, 63, a retired IT worker: “I certainly won’t protest. I’m sick of listening to it. People are just fed up and don’t understand it.”

It was a common sentiment: customs or no customs, do a deal and move on. “You’re not necessarily that happy with it but you want something that works for everyone,” said one woman, who declined to be named.

Julian Smith, Johnson’s Northern Ireland secretary, is spending the week at Stormont trying to break the political deadlock. A fraught challenge, but better than being trapped in a headlock.