The government wants to change the Northern Ireland Protocol to make it easier for some goods to flow from Great Britain to Northern Ireland.
Micheál Martin said they would damage Northern Ireland’s economy.
But DUP MP Sammy Wilson said Mr Martin was “focused on Dublin’s interests”.
Speaking on the BBC’s Sunday Morning programme, the taoiseach said that the legislation was unacceptable and represented “unilateralism of the worst kind”.
“We accept fully that there are legitimate issues around the operation of the protocol and we believe that with serious sustained negotiations between the EU and the UK government those issues could be resolved,” he said.
Mr Martin said other parts of the bill, namely dual regulatory standards, could be severely damaging to the Northern Ireland economy.
“It is deeply concerning to industry and businesses in Northern Ireland… in effect it represents a form of economic vandalism on Northern Ireland,” he said.
Mr Martin said data showed the Northern Ireland economy was “doing very well” under the protocol and argued that this was not being articulated enough by the UK government.
But the DUP’s MP for East Antrim, Sammy Wilson, accused the taoiseach of failing to recognise “the flaws of the protocol”.
“From day one Dublin has done what’s in Dublin’s interests and never once prioritised consensus in Northern Ireland,” he said.
“Not one unionist MP or MLA supports the protocol but instead of Dublin trying to understand or recognise our objections, they repeatedly lecture us, talk down to us and demand we change our mind.
He added: “It’s time for Dublin to step back and recognise that the reason we find ourselves without a fully operating devolved institution is because of their determination to steamroller over unionist objections.”
Sinn Féin’s vice-president Michelle O’Neill said it was an “irrefutable fact that the protocol is working”.
Speaking at the annual Wolfe Tone commemoration in Bodenstown, Co Kildare, Ms O’Neill said the UK government’s attempt to change the protocol “discards international law at a whim in such a reckless way”.
She said: “There is a majority in the assembly who support the protocol because it is working, giving us continued access to the European single market, creating jobs and economic benefit.”
Green and red lanes
The protocol prevents a hard border with the Republic of Ireland, but it means checks on some goods arriving into Northern Ireland from other parts of the UK.
The arrangement is opposed by unionists in Northern Ireland because it creates a trade border in the Irish Sea and they argue it undermines Northern Ireland’s place in the UK.
At the centre of the government’s plan to ease the impact on businesses is the concept of green lanes and red lanes for trade.
This would mean:
Goods coming from Great Britain (GB) into Northern Ireland (NI) and which are staying would use the green lane. This means there would be no checks and paperwork would be minimal.
GB goods moving through NI into Ireland or the wider European Union would use the red lane and continue to be checked at NI ports.
London also wants any trade disputes resolved by “independent arbitration” and not by the European Court of Justice, and Northern Ireland to benefit from the same tax breaks as elsewhere in the UK.
The government says there is “no other way” of safeguarding essential interests of the UK.
But the EU opposes the move, saying going back on the deal breaches international law.
Stena Line’s Irish sea ferry terminal at the company’s River Mersey Birkenhead dock on September 07, 2021 in Liverpool, United Kingdom
Draft legislation from the UK government this week to overturn parts of the protocol includes the use of a green lane for goods arriving into Northern Ireland from GB
Mr Martin said the EU had been flexible trying to come up with solutions to issues with the protocol.
The European Commission tabled proposals in October 2021 to reduce the post-Brexit checks on goods and medicines arriving into Northern Ireland from the rest of the UK.
At the heart of the plan was the idea of an “express lane”, to allow for a reduction in the level of paperwork and checks on goods coming into and remaining in Northern Ireland from GB.
But the proposals were publicly rejected by the UK in May.
London said they did not address the core problems presented by the protocol, would require more checks and controls than under the current “grace period” regime, and would result in the disappearance from Northern Ireland supermarket shelves of certain products.
What is the Northern Ireland Protocol?
The trade deal governs how goods enter Northern Ireland from the rest of the UK and was agreed by the UK government and the European Union following the Brexit vote in 2019.
It was designed to prevent a hard border on the island of Ireland when the UK left the European Union.
The protocol led to the creation of new goods checks at Northern Ireland sea ports on some products from Great Britain, effectively creating a new trade border in the Irish Sea.
Map of the the UK showing how goods travelling from GB into NI and onward to the Republic of Ireland.
Unionist parties, including the Democratic Unionist Party (DUP), argue that this has led to extra costs and unnecessary delays, as well as undermining the union between Northern Ireland and the rest of the UK.
Read more: A simple guide to the Brexit border problem
On Wednesday, the EU announced legal action against the UK government over its plans to scrap parts of the protocol.
European Commission vice president Maros Sefcovic said there was “no legal or political justification whatsoever for unilaterally changing an international agreement”.
“So let’s call a spade a spade, this is illegal,” Mr Sefcovic said, adding that the UK’s decision “left us with no choice” but to take legal action.
The NI Protocol and its economic impact
How will NI protocol plans affect businesses?
Prime Minister Boris Johnson’s spokesperson said his government was disappointed the EU had taken legal action and continued to favour a negotiated resolution.