The Taoiseach has said that some criticisms of the Northern Ireland Protocol by members of the British government are "bad-faith attacks".
Speaking at the plenary session of the European Parliament in Strasbourg, Michéal Martin said that while there are some reasonable criticisms of the protocol, some in the British government have invented issues with the agreement.
"It is perfectly reasonable to look for ways to improve the operation of the protocol – but unfortunately what we have seen are bad-faith efforts to undermine a treaty freely entered into.
"Instead of trying to create a constructive atmosphere for all to engage, we have actually seen efforts to block agreements or introduce new problems."
The Taoiseach said the EU has "worked hard to find ways to ensure that where there is a negative impact on businesses and citizens in Northern Ireland it is minimised as much as possible" in contrast to the UK's approach.
The protocol — part of the special post-Brexit arrangements for Northern Ireland to avoid a hard border on the island — has been an ongoing source of controversy, with unionists claiming it has effectively isolated the North from the rest of the UK by placing checks on the movement of some goods.
Mr Martin said the European Parliament has been "incredibly constructive" in trying to solve issues with protocol, pointing to April legislation which helped safeguard the supply of medicines to Northern Ireland.
The Taoiseach went on to say that British legislation to change the protocol would be a "historic low point" for Britain's international reputation for upholding the rule of law.
"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."
The Taoiseach also pushed the European Union to speed up Ukraine's accession to the organisation as well as Moldova and Georgia.
"We should support those looking to join in undertaking the reforms and preparations necessary, and when they meet the criteria we should not place further barriers in their way.
"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."
The Taoiseach added that Ireland is open to changing European treaties to allow the EU to form a "more active agenda".
"Our guiding principle should be how can we best prepare our Union to continue to meet the needs of its citizens into the future.
"Ireland is ready for this debate. We will work to constructively shape our new future and are open to treaty change if this proves necessary.
"We should first, however, do what we can within the existing framework."