Britain's new Prime Minister Boris Johnson has held talks in Northern Ireland in a bid to untangle an impasse over the Irish border backstop that has scuppered all efforts to secure an orderly withdrawal from the European Union.

Plans for the border have become the most contentious issue in negotiations with the EU, and the British pound has tumbled in recent days as Johnson said Britain would leave without a deal on October 31 unless the backstop was scrapped.

The head of Irish nationalist party Sinn Fein, Mary Lou McDonald, said she warned Johnson that leaving without a deal would be catastrophic for the economy and the 1998 peace deal that ended three decades of violence in the region.

Boris Johnson said there would be no hard border in Ireland. (AAP)

Sinn Fein president Mary Lou McDonald and deputy leader Michelle O'Neill meeting protesters after speaking to Prime Minister Boris Johnson at Stormont House in Belfast. (AAP)

Some 3600 people died in sectarian violence commonly known as The Troubles.

Johnson began his trip with talks on Tuesday evening with the leadership of the Democratic Unionist Party, the largest pro-British party in the region whose 10 members in the Westminster parliament prop up the Conservative government.

After the meeting DUP leader Arlene Foster repeated Johnson's demand that the backstop, designed as an insurance policy to prevent border controls between Ireland and Northern Ireland, be scrapped. "It is very important that the backstop goes," she said.

But a senior DUP lawmaker also at the meeting said possible compromises were discussed - specifically the possibility of putting a time limit on the backstop and other "pragmatic solutions."

DUP Leader Arlene Foster. (AAP)

Asked if Johnson was responsive to the suggestion, Donaldson told Irish radio RTE that he would not "negotiate in public on this."

Speaking to journalists ahead of the talks, Johnson said Brexit would be on the agenda, but he wanted a quick restoration of Northern Ireland's suspended power-sharing executive. It is a critical part of the 1998 Good Friday peace agreement that ended 30 years of conflict.

Prime Minister Boris Johnson with Northern Ireland Secretary Julian Smith (AAP)

The power-sharing administration was suspended two-and-a-half years ago because of differences between the parties representing mainly Protestant pro-British unionists and mainly Catholic nationalists who favour a united Ireland.

"The people of Northern Ireland have been without a government, without Stormont for two years and six months so my prime focus this morning is to do everything I can to help that get up and running again," Johnson told journalists.