The UK government has said that it "profoundly regrets" a decision by the Government to legally challenge the Northern Ireland Legacy Act.
Tánaiste Micheál Martin confirmed that the Government decided to initiate an inter-state case against the UK under the European Convention of Human Rights.
In a statement, Northern Ireland Secretary Chris Heaton-Harris described the case as "unnecessary" and comes at a "particularly sensitive time in Northern Ireland".
"It did not need to be taken now, given the issues are already before the UK courts," he said.
"While this step is disappointing, it is one for which the UK Government was prepared. The UK Government remains confident that the Act provides a robust and effective framework to allow the ICRIR (Independent Commission for Reconciliation and Information Recovery) to discharge our legal obligations.
"We will continue robustly to defend the legislation, including to ensure that the work of the ICRIR can continue without impediment while proceedings are ongoing."
In a statement earlier, Mr Martin said that the decision was taken after much thought and careful consideration, adding, "I regret that we find ourselves in a position where such a choice had to be made".
In its application, the Government will argue that the provisions of the Northern Ireland Troubles (Legacy and Reconciliation) Act 2023 are incompatible with the UK's obligations under the convention.
Mr Martin said the "decision by the British government not to proceed with the 2014 Stormont House Agreement and instead pursue legislation unilaterally, without effective engagement with the legitimate concerns that we, and many others, raised left us with few options.
"The British government removed the political option, and has left us only this legal avenue."
The Tánaiste added: "The incorporation of the European Convention on Human Rights into Northern Ireland law is a specific and fundamental requirement of the Good Friday Agreement. Since the UK legislation was first tabled, the Government have been consistent that it is not compatible with the Convention.
"I used every opportunity to make my concerns known, and urged the British government to pause this legislation."
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Mr Martin said that "serious reservations" about the bill have been raised by a number of international observers, including the Council of Europe's Commissioner for Human Rights and the UN High Commissioner for Human Rights.
He said the legislation is also opposed by people in Northern Ireland, especially the victims and families who will be most impacted by it.
Mr Martin said the Government has particular concerns around provisions "which allow for the granting of immunity, and which shut down existing avenues to truth and justice for historic cases, including inquests, police investigations, Police Ombudsman investigations, and civil actions."
He said that "even in cases in which immunity is not granted, 'reviews' by the proposed body, the Independent Commission for Reconciliation and Information Recovery (ICRIR) are not an adequate substitute for police investigations, carried out independently, adequately, and with sufficient participation of next of kin."
The Tánaiste said the British government enacted the legislation on 18 September "shutting off any possibility of political resolution".
He added: "We now find ourselves in a space where our only recourse is to pursue a legal path.
"It is important to leave the next steps to the court."
Taoiseach Leo Varadkar said the Government had a "sense of regret" in taking its legal action against the Legacy Act.
He said they never wanted to be in this position, but the Government had promised it would stand by victims' families and the Good Friday Agreement.
Govt 'doing the right thing' for victims - Amnesty
SDLP leader Colum Eastwood said confirmation of the move was "welcome" and "utterly necessary".
He said "no political party or institution on this island supports" the act, describing it as "the most extreme legislation that shuts down access to justice for victims and survivors".
"The inter-state case being taken by the Irish Government is both welcome and utterly necessary.
"If the British government will not respect the treaties it enters into, respect international law or respect the basic tenets of justice then it must be challenged robustly," said Mr Eastwood.
The Government has been praised by Amnesty International as "doing the right thing" for victims.
Amnesty International UK's Northern Ireland deputy director, Grainne Teggart, said it was important that the Irish Government takes this stand.
"The Irish Government is doing the right thing for victims, for the rule of law and for the upholding of human rights," she said.
Additional reporting PA
