Sinn Fein has accused DUP leader Arlene Foster of the "same old arrogance" after she called nationalism "narrow and exclusive" during a speech in England.
Mrs Foster also said Dublin's attitude had hardened since Leo Varadkar became Taoiseach last year as she gave a wide-ranging speech at a Policy Exchange event titled The Union and Unionism in London yesterday.
She told the crowd that unionism was inclusive in comparison with nationalism.
"Unionism is at its best when it is confident, outward looking and welcoming. We must move forward with confidence," said Mrs Foster. "Citizenship and rights are essentially unionist issues. They are issues we should set out to reclaim.
"Nationalism is by its nature narrow and exclusive. Being a unionist is the opposite.
"Unionism stands for pluralism and multi-culturalism. We are inclusive and welcome all.
"Confident unionism can capture the diversity that nationalism cannot. It transcends nationalism, and allows individuals to express the cultural values or identity they wish.
"The surest way to cement the Union is for Northern Ireland to be open and provide a successful environment in which to live and work. A Northern Ireland which embraces differing cultures and where minorities feel valued is one that few will choose to abandon."
The DUP leader hit out at the Irish government as she reiterated her party's commitment to Brexit and stressed that there was no desire for a hard border.
She said: "Our worry as unionists has been... the very, very aggressive nature of the Irish government. That has been a change, it has to be said, from the last government, from Enda Kenny's government.
"It has been quite aggressive. And that leads a lot of unionists in Northern Ireland to think, 'Is this just about the European Union, or is it about something else? Is it about trying to claim the fourth green field in terms of Northern Ireland?'
"As a unionist, I see no logic or rationale for a hard border being created between Northern Ireland and the Republic of Ireland. The only people stirring up the myths of border checkpoints are those who are committed to unpicking the Union. They seek to use such imagery to advance and build support for their long-term political objective."
Mrs Foster again insisted her party would not accept a Brexit deal that put a regulatory border down the Irish Sea.
She added a 'backstop' option of extending membership of the customs union, to allow for an alternative to be found, would need to be time limited.
Mrs Foster said: "There has to be a backstop on the backstop. It is vitally important that if that is to happen, there is clarity in relation to the timing of it, for businesses in particular.
"We will not support a withdrawal agreement that creates a legal protocol with a new regulatory border down the Irish Sea."
Describing her speech as a "missed opportunity", South Belfast MLA Mairtin O Muilleoir said Mrs Foster had produced "no new thinking, but more of the same old arrogance".
"Unfortunately, this much-trumpeted speech from Arlene Foster has failed to provide any indication that the DUP is prepared to end its blockage of the power-sharing institutions," the former finance minister said.
"In her speech, Arlene Foster accused nationalism of being 'narrow and exclusive' while lauding unionism as standing for 'pluralism and multi-culturalism'.
"Quite apart from the arrogance of dismissing the entire nationalist population in this way, Arlene Foster's comments quite simply don't stack up to the reality. Where is the multi-culturalism for Gaelic speakers who are still being denied equal language rights by the DUP?
"Where is the pluralism for the members of our LGBT community who are still being denied marriage equality by the DUP?," Mr O Muilleoir asked.
SDLP Brexit spokeperson Claire Hanna said she was disappointed that Mrs Foster "continues to try to sectarianise Brexit and the legitimate concerns people have".
"With a credible poll showing 69% of people here would vote to remain if there was another referendum now that the implications of Brexit are much clearer, Arlene Foster needs to get her head out of the sand," she said.
"Instead of using speeches to drive people apart, the DUP leader should be focused on getting back to work. No matter how much the British Government, the DUP or any Brexiteer wants to spin it - the only solution to protect our socio-economic and political interests on this island is to ensure full alignment with the Single Market and the Customs Union and this speech was a missed opportunity for Arlene Foster to clearly state that."