Irish people in the UK are less likely to identify themselves as British than almost any other ethnic group, according to information released in the 2011 census.
Gypsies, Caribbeans and Pakistanis are more than twice as likely to identify as British than are the Irish, while fewer than 40% of Irish people identify as British compared to more than 80% of many other ethnic groups.
The only ethnic group with a lower percentage who identify as British are "Other White".
Though many of these are recent immigrants from Europe, compared to the Irish who have been based in the UK for hundreds of years.
Chris Smith, of the Office of National Statistics, said Irish immigrants had maintained a sense of national identity throughout their time in Britain.
He said: "Irish migration goes back to the 1840s, it was 9%, 10% of the population of some parts of England in the 1880s, 1890s: very, very high proportion.
"But they seem to maintain their sense of national identity despite the length of time here."
Mr Smith added that many "Other White" immigrants would have no need to get UK citizenship.
He said: "The other white category would be mainly the Eastern Europeans, so the Poles, Lithuanians, people like that.
"The fact that most of the Eastern Europeans have come in the last decade obviously affects their perception: they don't feel British, they feel Polish.
"And there's no need for them, for example, to get UK citizenship because they have the same rights in this country without.
"A Polish passport is fine: you can work, you can live here, so there's no need to become a UK citizen."
Irish people who were born outside the UK but live in Britain were also less likely to hold a UK passport.
More than 91% of Irish people born outside of the UK do not have a UK passport, compared to 45% of Indians born outside of the UK and 31% of Pakistanis born outside of the UK.
Of the 579,000 Polish immigrants born outside the UK, 96% do not hold a UK passport.
In total, 7.5 million people in the UK were born abroad, making up 13% of the population.
Almost half of these immigrants, 3.4 million, hold a UK passport.
The census breakdown also showed more Europeans settled in Britain from 2001 to 2011 than in the previous five decades put together, with Europeans making up a higher proportion of immigrants than people from the Middle East and Asia.
Figures showed that 1.54 million moved to the UK in the decade to 2011, compared to 880,000 between 1961 and 2000 and a further 322,011 who arrived before 1961.