The Taoiseach Micheál Martin says unilateral action by the UK on the Northern Ireland Protocol would "mark a historic low-point".
Earlier Mr Martin addressed the European Parliament in Strasbourg, reflecting on 50 years of Irish membership of the bloc.
He told MEPs that EU membership had been "transformative" for the country.
While on Brexit, he said unilaterally changing the Northern Ireland Protocol benefits no one.
"I have said many times that there are solutions to practical problems under the protocol if there is a political will to find them.
"But that requires partnership. It requires the UK government to engage with good faith, seriousness, and commitment.
"Unilateral action to set aside a solemn agreement would be deeply damaging.
"It would mark a historic low-point signalling a disregard for essential principles of laws which are the foundation of international relations.
"And it would, quite literally, be to the benefit of absolutely no one."
While President of the European Parliament, Roberta Metsola, reiterated that the protocol is a closed chapter.
"Renegotiating the protocol on Northern Ireland is not an option.
"The EP has reiterated its unwavering support for the protocol on several occasions, and we remain fully committed to preserving peace on the island of Ireland", she said.
'Strongly support Ukraine's application'
On Ukraine, Mr Martin said he hopes it will be a 'positive message' on their EU membership bid.
"I strongly support Ukraine's application for membership and I look forward to the commission's opinion.
"I hope that, when the European Council meets in June, it will be possible to send the people of Ukraine a clear and positive message.
"Ireland's lived experience - and we are not alone in this - is that EU membership is transformative.
"It is not only about access to markets and to economic opportunity, though that is - of course - important, it is about what it means for people."
President Metsola also commended Ireland "on being the first country in the EU to waive visa requirements for Ukrainians, and for welcoming nearly 30,000 displaced Ukrainians who have sought shelter."
