Micheál Martin has accused the British government of moving “too far in a unilateral way” over the Northern Ireland Protocol.
Mr Martin also dismissed suggestions that the EU was being inflexible over the protocol and urged the DUP to rejoin the powersharing political institutions at Stormont.
The Taoiseach is in Belfast meeting with party leaders amid ongoing deadlock at Stormont over the post-Brexit trading arrangements.
He will also meet a range of business representatives on a visit that will be dominated by the political crisis over the contentious protocol.
The region’s main unionist party, the DUP, is currently blocking the re-establishment of Stormont’s powersharing institutions in protest at the protocol, which has created economic barriers on trade between Britain and Northern Ireland.
The Taoiseach’s visit comes in the wake of the British government’s controversial move to act unilaterally to scrap parts of the protocol.
Liz Truss announced on Tuesday plans to legislate to override parts of the Brexit withdrawal treaty it struck with the EU.
In response, Mr Martin told the BBC: “The European Union has said repeatedly that they can move on issues.
“I spoke to Boris Johnson and I have to nail this, this idea that somehow the European Union is being inflexible on this is just not the truth, it doesn’t stack up.
“What has happened now is a certain unilateralism on behalf of the British government saying ‘our way or no way’ and you don’t negotiate with the European Union on that basis, particularly when you have signed off on the agreement that you now don’t like.
“Professional, serious negotiations between the United Kingdom government and the European Union is the only way to resolve this.
“I believe that the current UK government has moved too far in a unilateral way on issues, be it legacy, be it the protocol.
“In my view that is not fully in accordance with the spirit of the Good Friday Agreement which involves collaboration, working together.”
Mr Martin also said there cannot be a situation where one political party is refusing to allow the Northern Ireland Assembly to meet.
He added: “I think most people would agree that in the democratic world when people vote for their representatives and vote to elect a parliament, the first thing that happens is that parliament should convene.
“It is unheard of in a democratic world that that Parliament would not convene in the aftermath of an election.
“We can’t have a situation where one political party determines that the other political parties can’t convene in a parliament.”
Speaking ahead of her meeting with Mr Martin, Sinn Féin vice president Michelle O’Neill accused the DUP of “denying democracy” by refusing to enter government in Northern Ireland.
Ms O’Neill said: “At a time where democracy is being denied, at a time where the DUP are continuing to prevent the facilitation of an Executive being formed, an executive that could start to deliver for the public, I think it is important that he is here to assert his role and to listen to all of the parties.
“There are parties here that want to be in government together, there are parties that want to be in the executive but unfortunately the DUP, sponsored by the British Government, are holding back all of that progress and preventing us from being able to start to put money in people’s pockets.”
Ahead of his meeting with the Taoiseach, DUP leader Jeffrey Donaldson insisted operation of the Stormont institutions would not have the consent of unionism while the protocol remained in place.
“Powersharing only works with the consent of unionists and nationalists,” he said.
“For two and half years every unionist MLA and MP in Northern Ireland has been voicing opposition to the protocol.
“There must be new arrangements if we are to move forward.
“We want to see the institutions working fully and relationships restored but that can only happen by building consensus.
“The unionist viewpoint can no longer be ignored.
“The protocol has damaged both Northern Ireland’s economic and democratic arrangements.
“It must be resolved or both our political and economic future will be bleak.
“It must be replaced by arrangements that can command the support of unionists as well as nationalists.
“We are happy to engage with the Taoiseach regarding the protocol and how our two countries operate on matters of mutual concern.
“The functioning of the Northern Ireland Assembly and executive, however, are entirely matters for the Northern Ireland parties and the UK government.”
The issue may have further ramifications for Britain, with US House speaker Nancy Pelosi calling the British government’s move to scrap parts of the protocol “deeply concerning”, adding that doing so will result in Congress not supporting a free-trade agreement between the two nations.
She wrote on Twitter: “It is deeply concerning that the United Kingdom now seeks to unilaterally discard the Northern Ireland Protocol, which preserves the important progress and stability forged by the accords.
“It continues to enjoy strong bipartisan & bicameral support in the United States Congress.
“As I have stated in my conversations with the Prime Minister, the Foreign Secretary & Members of the House of Commons, if the United Kingdom chooses to undermine the Good Friday Accords, the Congress cannot & will not support a bilateral free trade agreement with the UK.”
In response, Mr Donaldson told the BBC: “Nancy Pelosi states that the reason she is concerned is because the Good Friday Agreement might be undermined, but the protocol is undermining the agreement.
“That is absolutely evident.
“The protocol has changed some of the key principles of the Belfast Agreement and it has made it impossible to have powersharing on the basis of consensus because not a single unionist MLA elected to the Northern Ireland Assembly in the recent elections supports that protocol.
“If Nancy Pelosi wants to see the agreement protected then she needs to recognise that it is the protocol that is harming and undermining the agreement and that is why we need to deal with it.”