The Government has conveyed its concerns bilaterally through diplomatic and official channels amid reports that the British government may override parts of the EU-UK Withdrawal Agreement.
It is emphasising that the European Commission is clear that the Withdrawal Agreement and the Northern Ireland Protocol must be fully implemented as a precondition to any future deal.
It will now watch carefully, in consultation with the European Commission taskforce, whatever legislation may emerge from London in the days ahead.
Sinn Féin President Mary Lou McDonald called on the Taoiseach to contact the British Prime Minister to tell him that any such attempt is unacceptable to this country.
Speaking on RTÉ's Drivetime, Ms McDonald said: "It is pretty unnerving and disgraceful sabre-rattling if this is simply a message to make loud noise."
She was speaking after The Financial Times reported that Boris Johnson is planning legislation to override parts of the withdrawal treaty that Britain and the EU agreed last year.
The report cited three people close to the plans as saying a bill to be put before the British parliament this week would undermine agreements relating to Northern Ireland customs and state aid.
In response, Downing Street said only that it was still "working hard to resolve outstanding issues with the Northern Ireland Protocol" but was considering "fall-back options".
Ms McDonald called for a "strong expression of the almost unanimity across the island of Ireland to protect our interests island-wide".
It is essential, she said, that it is understood that agreements are respected by all of the parties concerned.
She reiterated that there must be no "undermining, unravelling or questioning" of the Good Friday Agreement.
Ms McDonald said that Ireland cannot become what she termed a "pawn in this Tory Brexit" and she urged the Government to be steady on its feet at this crucial time.
Meanwhile, the President of the European Commission warned that Britain is legally obliged to respect the Withdrawal Agreement, which must form the basis of bilateral relations in future.
Ursula von der Leyen tweeted: "I trust the British government to implement the Withdrawal Agreement, an obligation under international law and prerequisite for any future partnership," the president of the European Commission said.
But Ms von der Leyen warned that, in Brussels' view, the clause - which would see the British province continue to follow some EU rules while maintaining an open border with Ireland - is essential.
The protocol on Ireland and Northern Ireland, she tweeted, "is essential to protect peace and stability on the island and the integrity of the single market."
The Minister for Foreign Affairs said if the UK were to introduce domestic legislation that undermines commitments made in the Withdrawal Agreement it would be a very serious breach of trust and would undermine current Brexit trade agreement negotiations.
Simon Coveney told RTÉ's News at One that the implementation of what has already been agreed between the EU and the UK is a prerequisite to agreeing a future trade agreement, as has been made clear by Ms von der Leyen.
He said that he hopes the Financial Times report is "false" and we should not overreact, but any suggestion that the UK is not going to implement what has been agreed would "fundamentally undermine trust between two parties in the efforts to get a trade agreeement in the weeks ahead that would avoid tariffs and quotas from 1 January next year".
Mr Coveney said the consequences of failing to secure a trade agreement are very serious for Ireland and for the UK and that "we need to avoid that and conclude a trade agreement by the middle of October".
He said it would then be possible to move on to work to ensure businesses "are prepared for the new realities of trading" between Britain and Ireland and the UK and the EU.
Mr Coveney said that he plans to bring a significant paper on preparations in this regard to Cabinet tomorrow.
Tánaiste Leo Varadkar said that as far as the Government is concerned the Withdrawal Agreement, including the protocol on Northern Ireland, is an international agreement.
Mr Varadkar said international agreements and treaties have to be honoured and they trump any domestic legislation that any country may pass.
He said that agreement is in place to make sure that we do not see the emergence of a hard border, which is something we all want to avoid.
He said we are into the last few weeks of negotiations and what the new trade arrangements are going to be between Britain and Ireland and at this point there is often a certain level of sabre rattling and posturing.
Anti-Brexit parties in the Stormont Executive issued a joint statement expressing concern about the report that the British government may override parts of the Northern Ireland Protocol.
They say such a move would be "entirely unacceptable" and would represent "a shocking act of bad faith".
The statement is signed by Deputy First Minister and Sinn Féin's leader in Northern Ireland Michelle O'Neill, SDLP leader Colum Eastwood, Alliance leader Naomi Long and leader of the Green Party in Northern Ireland Clare Bailey.
The four pro-Remain political parties in the North have written to Boris Johnson and Michel Barnier today, calling on the British Government to honour their commitments and prioritise our peace, economic development and stability.— Social Democratic and Labour Party (@SDLPlive) September 7, 2020
More from SDLP Leader @columeastwood 👇 pic.twitter.com/5Qg3Hn7sKX
They say the protocol provides vital safeguards and call on the British government to honour commitments it has made.
"It is entirely unacceptable to the Northern Ireland parties that the UK Government would seek to abandon these safeguards and mitigations, which we believe would amount to a serious betrayal of an existing International Treaty," the statement says.
"The economic and political impact on the island of Ireland, on the United Kingdom and above all on the people whom we represent would be devastating, long-lasting.
"It would represent a shocking act of bad faith that would critically undermine the Good Friday Agreement political framework and peace process and the UK's ability to secure other crucial deals to protect the Northern Ireland economy.
"We therefore call upon the UK Government to honour its commitments, and to now ensure the rigorous implementation of the Protocol, prioritise peace and stability in Northern Ireland, and work to secure a future economic partnership with their EU colleagues now and in the weeks ahead."
Britain's Agriculture and Environment minister George Eustace denied the British government intends undermining the Withdrawal Agreement and Northern Ireland protocol.
He said the British government is committed to implementing the Withdrawal Agreement and the UK is working with the EU through the Special Committee established by the Agreement to jointly agree the specific terms of the treaty.
He said when that process ends there may be some outstanding issues - what he called "loose ends" that may need to be implemented through UK legislation.
Additional reporting AFP