DUP MP Sammy Wilson has said he regrets calling Taoiseach Leo Varadkar a "nutcase" in an ouburst over Brexit and future EU negotiations.
The East Antrim representative made the remarks just days after his party leader delivered a conciliatory speech in Killarney which sought to repair strained cross-border relations.
Mr Wilson also branded the Republic's premier arrogant, naive and inexperienced during an interview with website Politico.
He said there had been a change of tone since Mr Varadkar succeeded Enda Kenny last June and he criticised the taoiseach for siding with EU negotiators.
In an address to the European Parliament on Wednesday, the Fine Gael leader said the UK could not "backslide" on commitments made on the Irish border.
"It was always our view at the very start of this process that the biggest ally we would have when it came to negotiating with the European Union was Dublin, and indeed that always was the impression we got when Enda Kenny (former taoiseach) was in power, but since this nutcase Varadkar has taken over that things have all changed," Mr Wilson said.
The taoiseach's office declined to comment on the outburst, which Mr Wilson later said he regretted.
In a statement, the DUP MP said: "Leo Varadkar's approach may make him popular in Brussels but it will eventually destroy Ireland.
"Upon reflection, I should have said Leo Varadkar's EU policies defy logic rather than the language I used. I regret my choice of language."
Tanaiste Simon Coveney was asked for his reaction to Mr Wilson's remark during a visit to Belfast to announce a fresh round of Stormont negotiations.
Mr Coveney would not be drawn on the East Antrim MP's "nutcase" comment but he stressed the need for a constructive relationship between his government and the DUP.
"I have been quoted publicly that we need to have a good working relationship with the DUP," he said.
Mr Wilson's DUP colleague and former Stormont economy minister Simon Hamilton said Mr Wilson was "never shy about telling you what he thinks".
"On this occasion he used language which he accepts was wrong and he regrets the language that was used and I think he has already expressed publicly his regret at using the language that he did.
"We look forward to engaging with the Irish government as part of these talks on issues that are pertinent and relevant to Irish government."
Mr Wilson's remarks were reported on the same day former DUP health minister Edwin Poots criticised a former Labour party special adviser for describing his party as having a "spectrum disorder".
The Lagan Valley MLA said journalist Kevin Meagher, who a decade ago was an adviser to secretary of state Shaun Woodward, had used an analogy that was "totally unacceptable".
