The Irish government is to begin a legal challenge against the UK government over its decision to offer immunity for Troubles-era crimes.
Critics argued the law gave an amnesty to killers and removed access to justice for victims.
Ireland will bring a case under the European Convention on Human Rights.
Victims and survivors have already taken the government to court in the High Court in Belfast.
The 30-year conflict cost more than 3,500 lives and left thousands more seriously injured.
The legislation also brought an end to all new inquests and civil actions related to the conflict.
In a statement, Tánaiste (Irish Deputy PM) Micheál Martin said: “This decision was taken after much thought and careful consideration.
“I regret that we find ourselves in a position where such a choice had to be made.”
He added that the UK government had “pursued legislation unilaterally” despite the concerns of the Irish government and others.
“We are not alone in our concerns. Serious reservations about this legislation have also 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,” Mr Martin added.
“Most importantly, this legislation is opposed by people in Northern Ireland, especially the victims and families who will be most directly impacted by this Act.”
Opponents, including victims groups and Stormont parties, have argued the legacy bill will remove access to justice.
The UK government has argued that the bill is an attempt to draw a line under the events of the past.
The Irish government has opposed the UK’s new legacy law right from its conception.
Victims’ groups had lobbied it repeatedly to take a case in Europe.
It now means the Troubles Act is being challenged on two legal fronts.
Bereaved relatives have gone to court in Belfast and Dublin is acting in Strasbourg.
A decision by the Irish government had been on the cards, but this is a big step which will not have been taken without evaluating political implications.
In September, Northern Ireland Secretary Chris Heaton-Harris said the Royal Assent of the Troubles legacy bill marked a “significant milestone as the government aims to deliver on our pledge to deliver better outcomes for those most affected by the Troubles, while helping society to look forward”.
He added that the bill offers “a real opportunity to deliver greater information, accountability and acknowledgement to victims and families, moving away from established mechanisms that have left far too many empty-handed”.
Tánaiste (Irish Deputy PM) Micheál Martin said the decision was taken after much thought and careful consideration
The legislation will lead to the establishment of an Independent Commission for Reconciliation and Information Recovery (ICRIR).
The aim of this new organisation is to help families find out more about the circumstances of how their loved ones were killed or seriously injured.
Self-confessed perpetrators who provide a truthful account of their actions to the ICRIR can be granted immunity from prosecution.
Mr Heaton-Harris said the commission was part of a move to “build a legacy process founded on integrity, expertise and fairness”.
Troubles victims’ campaigner Raymond McCord, whose son Raymond was murdered by loyalist paramilitaries in 1997, said: “Anyone in Northern Ireland who condemns or criticises the Irish government over their action is no friend of victims”.
Amnesty International said the Irish government was “doing the right thing” for victims of the Troubles by taking the case.
The human rights organisation the Committee on the Administration of Justice (CAJ) said it was “the right decision and a necessary one”.
Sinn Féin leader Mary Lou McDonald also 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 “not surprising” and it takes an “opposition attack line off the table” ahead of an election year.
“If both the United Kingdom government and the Irish government are not willing to fix the wrongs of the past then it will be the victims who suffer once again,” he added.
