The DUP has bluntly told Taoiseach Leo Varadkar he is "playing with fire" by threatening to veto the next phase of Brexit negotiations.
Sir Jeffrey Donaldson warned the latest "megaphone diplomacy" from the Fine Gael leader could threaten decades of improved relations between the Republic and the UK.
"If he wants to go down as the Taoiseach who delivered a hard border he is going the right way about it," the DUP MP added.
Mr Varadkar was also accused of trying to block Brexit for his own party political reasons ahead of the next Irish general election.
Sir Jeffrey's attack came after the Taoiseach - on a tour of the US-Canadian border - said he "doubted" the other EU states will allow the Brexit talks to progress to the next stage dealing with trade unless Dublin was happy with progress on three key issues.
These were listed by the EU as the Irish border, the so-called 'divorce settlement', and the rights of EU and UK citizens - all of which must be progressed satisfactorily before trade talks commence.
"That will be our decision," Mr Varadkar said. "We will decide whether the UK has made sufficient progress when it comes to citizens' rights, the financial settlement and issues pertaining to Ireland, and only if we are satisfied that sufficient progress has been made on those three areas will we then give the go-ahead to talks about trade.
"So I think that puts us in a strong position as I doubt our European colleagues would come to the view that sufficient progress had been made if we didn't think so."
But Sir Jeffrey blasted the comments as "very unhelpful" and added: "This appears to be further evidence that Dublin is not interested in co-operating in trying to find a pragmatic approach addressing ideas around customs and trade.
"It seems what they are dong is trying to block Brexit and that is very unhelpful."
The Lagan Valley MP said it seemed Dublin was determined to thwart the democratic will of the British people who voted last year to leave the EU.
"If so, that will raise serious questions over the good relations that have built up between the two countries in recent years," he added.
Sir Jeffrey said the Irish Government could exercise a veto but in the end the EU will decide on the need to make progress towards agreeing with the UK the terms on which it will leave.
"He (Mr Varadkar) is putting it up to Brussels as much as he is to London and he really needs to be careful," he added.
"He is playing with fire and posturing for the benefit of his own domestic audience, lining himself up for the next Irish general election."
Sir Jeffrey also said Mr Varadkar's comments did not make economic sense, with almost 50% of all Irish meat products going to the UK.
"How is he going to explain putting party political interests before the interests of Irish farmers?" the DUP man asked.
"It is an extension of the megaphone diplomacy we have seen from Dublin in recent times, only this time he has taken the megaphone to Canada."
Ulster Unionists also criticised Mr Varadkar for "playing politics" and insisted the Republic will be just one voice among the other 27 EU countries.
Steve Aiken MLA said: "Unfortunately, Leo Varadkar and Simon Coveney are yet again playing politics with very serious issues.
"Rather than looking at the suite of UK Government policy proposals and assessing clearly how this could support trade and relationships across all these islands, they yet again appear to be trying to set Dublin as the arbiter of the relationship between London and Brussels.
"Anyone with any understanding of the process would see it is clear that Ireland is just one voice of 27 and rather than carping from the sidelines, Ireland should realise it is in their interest - probably more than anyone else's in the EU - to secure an equitable and sustainable future relationship between the United Kingdom and the European Union."
The latest spat comes less than a month after the DUP accused Mr Varadkar of playing "high wire politics" with Northern Ireland over the form of a so-called hard border which both have said they want to avoid.
Mr Varadkar had said he did not intend to help devise a system for the border for Brexiteers, and that if the UK wanted technological solutions to the border it was a matter for it.