Named after campaigner Philomena Lee, the law stops people in Britain losing social welfare entitlements and other state supports if they receive a payment from the Irish Government's mother and baby homes redress scheme.
Up to 13,000 people living in Britain may be eligible to claim under the scheme, but so far only about a thousand have done so.
At the London Irish Centre in Camden, a small group gathered this morning to mark the successful passage of Philomena's law, which came into effect today - just a year and a half after beginning life as a private members bill, promoted by Labour MP Liam Conlon.
The law's aim is to correct a manifest injustice: survivors of Ireland's mother and baby homes could get financial redress under an Irish scheme, but many in the UK have been reluctant to take it because it would have counted towards their means tested income.
And that could have cost them social welfare and other state supports like care home payments - leaving them worse off.
Mr Conlon described the change as "massive" and "a historic day".
"It's the first time ever in British law that a payment from a foreign government has been ring-fenced, and it's all been about the 13,000 survivors of Ireland's mother and baby homes living in Britain.
"They received a long-awaited apology and then access to a redress scheme but were being penalised if they accepted that payment in Britain while receiving means-tested benefits.
"We have campaigned really hard for the last 18 months and I am so pleased that it's now law as of today."
The law is named after Philomena Lee, originally from Limerick, whose story of the forced adoption of her son from a convent was turned into an Oscar-nominated movie starring Judi Dench and Steve Coogan.
Her daughter and fellow campaigner Jane Libberton was in Camden for the event marking the laws start, and said: "I think its amazing. I think its a really good thing because i hope it allows more people to actually claim for the redress scheme.
"I don't think that many have, and I think a lot of people also don't know about it - that's part of the problem.
"So by Philomena's law coming in I hope they can publicize it for people to know that they can claim."
'Lasting legacy'
Philomena herself has recently moved into a care home, but Ms Libberton says she is delighted by the new law.
She said: "I spoke to her about it yesterday. She's 93 now and is getting a bit frailer, but she was absolutely delighted.
"It's just a shame she cant be here herself - she would love to have been. But it's a lasting legacy for her and her story - and many other people and their stories as well."
Mr Conlon says the movie was a big help in trying to build cross party support for his bill in the House of Commons and beyond.
"I think people in Britain, when you're trying to talk to them about this story, may not have heard of mother and baby homes, but they've all heard of Philomena Lee and the film Philomena," he said.
"People really connected with that and it helped to build support, build cross-party support and connect with the public.
"We've had an overwhelming level of support for this and it's really helped us get to where we are today."
Getting to this point happened rather quickly by Westminster standards.
Mr Conlon was only elected to the House of commons in 2024, winning his seat in Beckenham and Penge in greater London at the General Election that year.
He tabled Philomena's Law as a private members' bill in March of last year, getting ten minutes to introduce the bill to the House.
It helped greatly that he was able to garner cross party support for the bill.
Then the campaigning started in earnest with Irish community groups contacting MPs up and down the country.
"We didn't think we'd get it through this quickly when we introduced it to Parliament last March.
"It's been a testament to the Irish community in Britain, who have come together and worked really hard.
"MPs from across the political spectrum also came together to work on this, as did Pat McFadden, the Secretary of State for Work and Pensions, whose parents are from Donegal and who really understood the issue.
"When I went to see him, he immediately got it. He helped push it through and get us to where we are today."
In March of this year the then UK Prime Minister Keir Starmer backed the bill, as did Taoiseach Micheál Martin.
At a meeting in Cork the two leaders agreed to reciprocal measures, ringfencing the Irish redress scheme payments from British welfare law, and agreeing the same principle should apply to any similar British payments to people living in Ireland.
The original Irish legislation in the Redress Scheme for the Mother and Baby Homes already ringfenced redress payments from Irish welfare rules, so the same issues did not arise in Ireland.
In April the UK Department of Work and Pensions under Mr McFadden issued an advisory to national and local civil servants to disregard redress Scheme payments from calculations for welfare entitlements while legislation was pending.
Adopted as Government legislation, regulations to formally change the law were tabled in the Commons in September, and came into force today, Friday, 30 October.
For Irish community groups in Britain, the priority now is to tell people who may be eligible for the Redress Scheme about the change in the law that means they won't risk losing more than they gain.
Irish in Britain, an organisation that links Irish groups in the UK and which campaigned in favour of Philomena's law, said the Irish redress scheme is only open to new claims for another two and a half years, so there is an urgency in getting the message about the new law out to the community, especially in Scotland where there has been a notable lack of uptake of the scheme.
Rosa Gilbert, Policy and Research manager with Irish in Britain said: " Now that we have this law, the message is to ensure that survivors based in Britain apply to the scheme.
"They can come to places such as the London Irish Centre or phone the national freephone helpline to get support with their application.
"The payment scheme is about halfway through its lifespan, so there are only another two and a half years left for people to apply.
"The message is for people to come forward now. With their benefits now ring-fenced, they can apply without fearing that their housing benefit, pension credit or other means-tested benefits will be cut off."
And she pointed out that it is not just people who are currently claiming benefits that will benefit from the change in the law: " Even though it may appear that this only affects a small proportion of those who are eligible to apply to the scheme, anyone could need to rely on means-tested benefits in the future, whether that's for social care or other forms of support.
"This legislation therefore ensures that it is not only people currently claiming benefits who will benefit from it.
"It will also protect people who may need to claim benefits at some point in the future, with these compensation payments remaining ring-fenced".
Ms Gilbert also called on the Irish Government to publicise the change in the law to the Irish in Britain.
"In terms of reaching people who might not normally come forward to Irish centres, or who may not be part of an established Irish community, it is really important that the Irish Government carries out a public information campaign", she said.
"That means getting information onto radio programmes, into newspapers and through statutory services and GP surgeries, wherever people are likely to come across it.
"In particular, there needs to be a focus on reaching survivors of mother-and-baby homes and ensuring they are aware of the scheme, because lack of awareness is often the first hurdle people face in accessing it."
Mr Conlon agreed with the need for outreach, especially to those who are not connected to organised Irish networks in Britain.
He said: "Because of the potential to be penalised for accepting compensation in Britain, people haven't been coming forward to claim it.
"We've heard stories of people who came forward, made an application and then pressed pause.
"What's really good is that we now know, from speaking to groups like the London Irish Centre, that there are people already benefiting from this legislation.
"What we need to do now is contact thousands more survivors who are eligible, let them know about it and support them through the process.
"That's exactly what we're going to be doing."
The London Irish Centre in Camden operates a Freephone Line for callers within the UK only, on 0800 519 5519. the line is open from 10 am to 4 pm, Tuesday, Wednesday and Thursdays.