The Taoiseach has urged Prime Minister Boris Johnson to "steady the ship" and pull back from plans to override Brexit's Northern Ireland Protocol.
Micheál Martin said "unilateralism will not work" and would be "deeply damaging" as he urged the UK government to negotiate agreed solutions with the European Union.
He said Brussels would respond "in a calm and in a firm way" if London presses ahead with legislation at Westminster to remove parts of the Irish Sea trade deal.
Read more: Video: SDLP's Claire Hanna evokes infamous Will Smith Oscars quote in clash with DUP
It followed a warning from the Irish foreign affairs minister Simon Coveney that divisions within the Conservative Party should not dictate the UK's approach.
Mr Johnson survived a confidence vote on Monday night after 211 to 148 Tory MPs backed the Prime Minister.
Legislation from Foreign Secretary Liz Truss on the Protocol was expected to be published on Wednesday, but that date has slipped amid the Conservative Party's leadership turmoil.
Speaking at the European Parliament in Strasbourg, Mr Martin said he did not want to become involved in "what is transpiring internally within British politics".
He added: "As far as I'm concerned I'm dealing with the British Prime Minister, and I engage with the British Prime Minister Boris Johnson.
"I have appealed to him consistently to engage with the European Union, who have shown flexibility, will continue to show flexibility.
"I get a sense that people want a bit of stability, people want to steady the ship now, and I think the first thing we could do collectively - Europe and the UK and Ireland as members of the European Union - is to do the sensible and pragmatic thing, get into negotiations and resolve the issues around the Protocol."
The Taoiseach said unilateral action "will not work", adding: "I think that would be deeply damaging because it would represent the violation of international agreements.
"It really is very difficult to comprehend the United Kingdom government in this day and age contemplating that, but we shall see. We will wait and see what happens."
He added: "Europe will respond in a calm and in a firm way to whatever decisions are taken by the British government and in relation to whatever legislation is both published and then enacted."
The Taoiseach unveiled a sculpture of Nobel laureate John Hume as part of a series of engagements in Strasbourg marking Ireland's presidency of the Council of Europe.
The bronze bust of the former SDLP leader, a key architect of the Good Friday Agreement, marks his contribution during his 25 years as an MEP and the inspiration he took from the European project.
A violinist played Danny Boy as the bust was unveiled by Mr Martin alongside the President of the European Parliament, Roberta Metsola.
Among the dignitaries in attendance at the event were European Commission vice-president Maros Sefcovic and European Commissioner Mairead McGuinness.
Mr Martin said it was a privilege to unveil the artwork, adding that Mr Hume was a "powerful international voice for justice and for resolution, and for peace in Northern Ireland".
The late Mr Hume's son John jnr said he felt "quite emotional" attending the unveiling.
He told Belfast Live: "It's a huge honour. The city of Strasbourg and the European Parliament meant a huge amount to our dad.
"The symbolism of this place, it was a living example of how consensus politics works and works for everyone.
"It played a huge role in his life. It's not an overstatement to say it was a huge inspiration."
For months Northern Ireland has had no functioning Stormont Executive or Assembly due to the DUP blocking a return to power-sharing in protest over the Protocol.
Asked what his father would think of the current stalemate, John jnr said: "As dad would have probably said, it's time to get back round the table and work things out. Dialogue harms nobody.
"The Protocol has its faults but they can be all ironed out."
Read more: Video: SDLP's Claire Hanna evokes infamous Will Smith Oscars quote in clash with DUP
Read more: John Hume sculptor on the "privilege" of creating bronze bust in his memory
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