Loyalists across Northern Ireland will light towering bonfires on Sunday night to commemorate the fires that helped guide William of Orange’s fleet to Carrickfergus, and victory, in the Battle of the Boyne. The following day, the Orange Order will hold more than 100 parades across the region celebrating the same 1690 battle.

The celebrations have long been the source of sectarian conflict between the mostly Protestant loyalists who favour Northern Ireland’s union with Great Britain, and the mostly Catholic nationalists who favour a united Ireland.

But fears of trouble are particularly acute this year, as the loyalist community reels from the aftershocks of the UK’s departure from the EU and post-Brexit trading arrangements, which have effectively created a trade border in the Irish Sea.

Even a temporary deal to ease the post-Brexit trading barriers between Northern Ireland and Great Britain has done little to calm tensions as the region heads into the year’s most contentious weekend of loyalist celebrations.

“Many people who were angry previously are still angry. They see this as just a case of kicking the can down the road for three months and therefore it has no effect,” Doug Beattie, leader of Stormont’s second biggest unionist party, the Ulster Unionist party, said.

The deal, agreed on June 30, includes a further three-month delay to checks on chilled meat entering Northern Ireland from Great Britain, as well as permanent measures making it easier for medicines, guide dogs, livestock and motorists to move between Northern Ireland and Great Britain.

The changes will smooth the implementation of the Northern Ireland protocol, part of the UK’s Brexit treaty that allows the region to remain in both the UK and the EU’s common market for goods. But they found little favour with loyalists who lined Belfast’s Shankill Road for one of the biggest pre-July 12 parades last weekend.

“It shows the politicians aren’t listening,” said Harry Shaw, 60, who lives in the Shankill area. He argued that no amount of tinkering with the protocol would make it acceptable, because “we’re British citizens. We’re getting treated as if they [the EU] are in control.”

Many of his sentiments were echoed by others of a similar age at the parade, highlighting how dissatisfaction extends beyond the young people involved in April’s riots. Isaac Cosby, 56, who marched with a local band, said he would oppose even a softened version of the protocol since “there should be no border in the Irish Sea”. Asked what he didn’t like about the protocol, William Shaw, a 55-year-old from Belfast’s loyalist Black Mountain area wearing a jacket with military badges, replied “everything”.

With opposition to the protocol running so high, political leaders are keen not to agitate the situation with talk of violence that could become self-fulfilling.

Beattie argued that an alternative to the protocol, which he has been advocating for years, is available. This is a system where border checks could be avoided with broader use of trusted-trader schemes to exempt some importers from checks, and by stamping goods for use in Northern Ireland only and making it a legal offence to export them. He hopes the extension will offer an “opportunity” to try to win support for his idea.

Jeffrey Donaldson, the newly installed leader of the larger Democratic Unionist party, told UTV that he would “encourage people not to engage in anything that might be seen as provocative”. He added: “I don’t think that burning posters, whether it’s loyalists or republicans or anyone else, is a way of being respectful.”

Donaldson has made veiled threats to collapse Northern Ireland’s power-sharing government if the protocol isn’t revoked, a development with potential to destabilise the region.

The EU has shown little inclination to budge from its position that the protocol is a fundamental part of the Brexit withdrawal agreement and must be implemented, despite the UK’s increasingly loud calls for more flexible arrangements.

“There’s a feeling to some extent that efforts . . . to mitigate the protocol might be enough [to satisfy the DUP],” Jon Tonge, professor of politics at the University of Liverpool, said. He added that the nuanced approach of removing most of the border checks, while keeping the principle of the protocol, could allow “honour to be saved on all sides” in the medium term.

In the meantime, he expects loyalists to speak through their bonfires: “You’ll probably see some pretty ugly demonstrations on the 11th, effigies of [EU Brexit commissioner Maros] Sefcovic, [Irish foreign affairs minister] Simon Coveney, [US president] Joe Biden. They’ll all be on bonfires somewhere,” he said.

Some bonfires have already led to conflict in communities. On Wednesday night Sinn Féin Stormont member Carál Ní Chuilín called for an “urgent meeting” with Northern Ireland’s health and education ministers, claiming activity around a loyalist bonfire site in Tiger’s Bay was leaving children in the neighbouring nationalist New Lodge area “terrified in their own homes and afraid to go outside”.

Jonathan Powell, chief adviser to UK prime minister Tony Blair in 1998 when the Good Friday peace agreement was struck, said people “here in London and Westminster and Whitehall are underestimating how dangerous it is, and likewise in Brussels”.

“I don’t think this means we go back to the Troubles, but I can’t rule out that there will be more rioting,” he said.

Last weekend’s march on the Shankill Road, when thousands of loyalists commemorated murdered Ulster Volunteer Force leader Trevor King, passed off with the air of a fete. Young children banged Union Jack drums and sprayed party string along the footpaths.

Still, with his grandchildren playing under the watchful eye of his wife Pearl, Harry Shaw said he was “very concerned” that unrest over the protocol “could plunge us right back to the ‘70s”. That was the height of three decades of sectarian conflict when more than 3,600 lives were lost.

A generation below him, Graham Foster, a 37-year-old from Ballysillan in north Belfast who marched with the Pride of Ardoyne band, said trouble this summer is “not something I want to think about”.

“I’m a family man,” he said, with his young daughter sitting nearby. Still, he added that, while he didn’t want to fight, “talking is not getting us anywhere, to be honest”.