The DUP has described a meeting with the Irish Foreign Minister about the Brexit deal as "useful but frank".
Gregory Campbell was speaking at Stormont, where Simon Coveney has held discussions with political leaders.
Earlier, the tánaiste said the DUP had turned down an initial offer to meet him during his visit to Belfast. Mr Campbell said until Thursday, "no specific request" had been made.
Mr Coveney said the meeting had been open and robust, but respectful.
Mr Campbell, MP for East Londonderry, told reporters that if the Irish government was not clear on the DUP's opposition to the withdrawal agreement before, "they have no doubt now".
The DUP - and other parties across Parliament - have vowed to vote down the deal in the Commons next Tuesday.
Theresa May has been trying to offer assurances to MPs over the Irish border backstop, but so far they have been dismissed.
Mr Campbell said unless substantial changes were made, the deal would fall - but that the DUP would keep the door "open" for further discussions on a way forward.
He accused Mr Coveney of "clinging" on to the backstop, when the DUP felt it is unnecessary as commitments have been made by governments to avoid a return to a hard border on the island of Ireland.
He described the backstop as a "bogeyman".
Mr Coveney's government has rejected UK proposals to give the Stormont assembly a say over new EU rules for NI, if the border backstop came into effect.
The Irish government has urged support for Theresa May's Brexit deal, despite criticism from the DUP and other parties at Westminster over the backstop proposal.
"We want to be helpful to the prime minister and helpful to the process, but we can't undermine the deal struck with the EU, and the legal text of the withdrawal agreement," Mr Coveney said.
"There has to be a fine balance."
Speaking earlier on BBC Good Morning Ulster Mr Coveney said: "We may not be convincing [the DUP MP] Sammy Wilson but that's not the same as convincing unionism.
"We're not trying to bypass the DUP but they cannot have a veto over this process."
Mr Coveney also said the DUP did "not speak for all of unionism".
He added that he was not "trying to meddle" in the affairs of the British government, but that he had an obligation to protect the interests of the island of Ireland as a whole.
Sinn Féin also met with Mr Coveney at Stormont.
Vice President Michelle O'Neill said there could be "no room for renegotation" on the withdrawal agreement.
"It is crucially important that the tánaiste and the Irish government continues to stand up for the people here in the north," she said.
Earlier on Thursday, Naomi Long said Parliament needs to "stop chasing unicorns" and get its act together on Brexit as the vote looms closer.
Mrs Long said there were now only two options on the table.
She said either parliament backed the current deal - and the Irish border backstop - or sought the revocation or extension of Article 50.
She also said there was no such thing as a "managed no deal" Brexit.
The SDLP leader Colum Eastwood said Labour needed to change its stance on the backstop.
Jeremy Corbyn has said EU withdrawal agreement amounts to a "de facto Irish Sea border" and has vowed his MPs will vote it down next week.
Mr Eastwood said there now needed to be a full defence of the backstop, which was part of a "sensible" deal for Northern Ireland.
MPs have been debating the withdrawal agreement ahead of a Commons vote next Tuesday, which the government is expected to lose.