DUP leader Arlene Foster has said Dublin's "very aggressive" Brexit stance has left unionists wondering if it is an attempt to claim Northern Ireland.
Mrs Foster also hit out at former prime ministers Tony Blair and Sir John Major over the positions they have taken on the Northern Ireland border issue.
Speaking at a conference on unionism in London, the DUP leader dismissed as "myths" claims Brexit would see a return to a hard border with checkpoints if the UK leaves the customs union.
The DUP leader said Dublin's attitude had hardened since Leo Varadkar became Taoiseach last year.
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."
The DUP leader also said it was "very wrong" for people like Mr Blair and Sir John to emphasise potential downsides of Brexit and not respect the vote to leave.
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."
She said she hoped the Northern Ireland Assembly would be back in operation "very soon" so it could decide on devolved issues.
Tory former Northern Ireland secretary Theresa Villiers also told the conference is was possible to avoid a hard border and leave the customs union.