How many Britons are entitled to an Irish passport?
Following a surge in post-Brexit citizenship requests, it was widely claimed that a quarter of British people have Irish heritage. What's the true figure?
Lots of news reports - including in the Daily Express, the Daily Mail, the Sun and even the BBC - have given a figure of one in four British people having Irish heritage.
That equates to 16 million people. But this turns out to be one of those statistics that may not be totally accurate.
There was a question in the 2011 Census about ethnicity that gave people in England, Wales and Scotland the option of ticking a "White Irish" box.
But only about one in 100 people ticked that box - a lot fewer than one in four.
In a roundabout way, however, the number is connected to that "White Irish" box on the census form.
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It was first added as an option on the census form in 2001, the result of a push by Irish groups in Britain for their ethnicity to be formally recognised.
And in early 2001, people with Irish links were encouraged by various advertising campaigns to "Tick the box" and to "Be Irish, Be Counted".
Around the same time, Guinness commissioned the polling company ICM to do a survey asking people if they had any Irish ancestry or saw themselves as Irish. One in four said yes, and that's where the figure actually comes from. It was reported by the BBC at the time.
Of course, claiming Irish heritage in a survey isn't the same thing as being entitled to an Irish passport. So the "one in four" figure isn't that useful.
At this point I have to own up to a personal interest in this story. With all the coverage around Britons applying for Irish passports, a friend pointed out that I might be entitled to one - something I'd never even thought about before.
So do I have what it takes to be an Irish citizen? And how many others out there could also be latent Irish citizens?
I put in a call in to Ireland's Citizens Information service and explained that my mother was born in Belfast but had never held an Irish passport. I was born in England and have also only ever had a British passport.
Is that enough to qualify?
"Yes," says Sharon Dillon, of the Citizens Information service.
"You would have an automatic right to Irish citizenship, which would then in turn entitle you to apply for and carry an Irish passport. Because your mother was born on the island of Ireland, she is recognised as an Irish citizen. She would be entitled to dual nationality - both British and Irish citizenship. That has been recognised and enshrined within the terms of the Good Friday Agreement."
The same rules apply if you have a grandparent born on the island of Ireland.
There are a few other subtleties to the rules on getting Irish citizenship, including a change for people born after 2005.
But broadly speaking, to work out how many British people are actually entitled to an Irish passport, we need to know how many people with at least one Irish parent or grandparent there are in the UK. You might think that's just a matter of looking at data from the census, but there's a hitch.
"It's not been asked in the census except in one year, in 1971, when parents' birthplace was asked," says Bronwen Walter, emerita professor of Irish diaspora studies at Anglia Ruskin University in Cambridge. "But it wasn't asked before and it hasn't been asked again."
Walter is one of the co-authors of a detailed study into the Irish diaspora commissioned by the Irish government.
Using that 1971 information, Walter and her colleagues came up with a number for second-generation Irish immigrants in Britain, trying to account for things like birth rates and changes in the number of Irish immigrants marrying outside their community.
"I think a very conservative estimate would be 1.7 million people very definitely would have at least one Irish-born parent, but it's quite likely to be higher," she says.
Calculating the number of third-generation immigrants is even harder, but the study authors were able to come up with an estimate using a longitudinal study.
"It was a 1% sample of the 1971 census that was extracted of people born in one particular week in 1971," explains Walter. "And in the 1981 census those exact people were brought out again to see what had changed for them and again 1991 and that has continued."
Using that data, they found "of people under 16 living at home, so this will be quite a serious underestimate, we worked out 3.4 million people definitely had an Irish grandparent in that study".
Walter's study was published back in 2001, but we haven't been able to find any similarly thorough research published in the time since.
So is the study still valid?
"There's a very strong influence of the 1950s large immigration from Ireland, people who are now aged say 55 to 65, and they continue to be a very large part of the second-generation population," says Walter.
"And of course they have children, who are the grandchildren. So I think there's quite a stable population, but added on to that there are many changes about migration, about family size."
But she thinks those changes roughly balance each other out in terms of the overall numbers.
So, roughly five million second and third-generation Irish people are in Britain. Without even looking at first-generation immigrants, that's already about 10 times as many as ticked the "White Irish" ethnicity box in the last census. So why the discrepancy?
"British came first and many people thought this was a question about nationality," explains Walter. "I think another reason was that being second-generation Irish has been very difficult for people in both Britain and Ireland. Because they don't have an Irish accent, they're frequently told they're not Irish. They're making a fuss if they say they are.
"And I think another reason is there's been a lot of out-marriage. So a far greater proportion of people who have an Irish parent will also have an English parent or someone of a different ethnicity. They're being asked to pick just one. So they don't necessarily pick Irish.
"But we did have a discussion group with people in all parts of the country after the 2001 Census and many people said, 'Oh, if I'd realised it was me, I would have ticked. I didn't realise it was for me.'"
If we leave nebulous questions of ethnic identity to one side, we can take Walter's estimates for the second and third generations and add numbers from the most recent census about people entitled to Irish passports to get a grand total.
Looking at first-generation migrants, there are about 650,000 people in England, Wales and Scotland born on the island of Ireland. Subtract the number of people who already have an Irish passport and you get about a quarter of a million.
Then we come to Northern Ireland. There may be people there who would choose not to take up Irish citizenship for any number of reasons, but if we focus just on the entitlement to an Irish passport and again subtract those who don't already have one, that's another 1.3 million people.
That means about 6.7 million people in the UK who don't already have an Irish passport who could be entitled to one. We think that's a conservative estimate, but it's still quite a lot more than the current population of the Republic of Ireland, which is 4.8 million.