The Irish government is set to launch a legal challenge against the UK government to grant immunity for Troubles-era crimes.
Critics argued that the law gave amnesty to killers and deprived victims of access to justice.
Ireland will continue to sue under the European Convention on Human Rights.
Victims and survivors have already taken the government to court at the High Court in Belfast.
More than 3,500 lives were lost and thousands were seriously injured during the 30-year conflict.
The act ended all new investigations and civil actions related to the conflict.
In a statement, Tánaiste (Irish Deputy Prime Minister) Michael Martin said: “This decision was made after much thought and careful consideration.
“I am sorry to find ourselves in a position where such a choice has to be made.”
He said the UK government “unilaterally followed the law” despite concerns from the Irish government and others.
“We are not alone in our concerns. Serious reservations about the law have been raised by many international observers, including the Council of Europe Commissioner for Human Rights and the UN High Commissioner for Human Rights,” Mr Martin added.
“Most importantly, this legislation is opposed by the people of Northern Ireland, particularly the victims and families who will be directly affected by this legislation.”
Opponents, including victims’ groups and Stormont parties, argued the Legacy Bill would remove access to justice.
The UK government argued that the bill was an attempt to draw a line under past events.
The Irish government has opposed the UK’s new succession law since its conception.
Victims’ groups repeatedly lobbied to take a case to Europe.
Now the issues statute is being challenged on two legal fronts.
Bereaved relatives have gone to court in Belfast and Dublin is playing in Strasbourg.
A decision by the Irish government was on the cards, but it was a major step that could not have been taken without assessing the political implications.
In September, Northern Ireland Secretary Chris Heaton-Harris said, “The Royal Ascent of the Troubles Legacy Bill is a significant milestone as the Government aims to deliver on our promise to deliver better outcomes for those most affected by the Troubles. Going forward”.
He said the bill “offers a real opportunity for more information, accountability and recognition for victims and families, moving away from established mechanisms that leave many empty-handed.”
Tánaiste (Irish Deputy Prime Minister) Michael Martin said the decision was made after much thought and careful consideration.
The Act will lead to the establishment of an Independent Commission for Reconciliation and Information Recovery (ICRIR).
The purpose of this new organization is to help families learn more about the circumstances of how their loved ones were killed or seriously injured.
Self-confessed criminals who provide ICRIR with a truthful account of their actions may be exempted from prosecution.
Mr Heaton-Harris said the commission was part of a move to “create a legacy process based on integrity, professionalism and fairness”.
Raymond McCord, a campaigner for the victims of Raymond’s murder by loyalist paramilitaries in 1997, said: “Anyone in Northern Ireland who condemns or criticizes the actions of the Irish Government is no friend of the victims”.
Amnesty International said the Irish government was taking up the case and “doing the right thing” for victims of the Troubles.
Human rights body the Commission on the Administration of Justice (CAJ) said it was a “correct decision and a necessary one”.
Sinn Féin leader Mary Lou McDonald welcomed the Irish government’s move, as did SDLP leader Colum Eastwood.
Ulster Unionist Party leader Doug Beattie said the Irish government’s decision was “no surprise” and would take the “opposition line of attack off the table” ahead of an election year.
“If the United Kingdom Government and the Irish Government are not prepared to correct the mistakes of the past, the victims will be victimized again,” he added.
