The number of Irish-born people in the UK is nearly 100,000 higher than Ireland's UK-born population, official analysis shows.
On average 375,900 people born in Ireland were living in the UK from January 2013 to December 2015, compared with 277,200 UK-born occupants in Ireland in 2016.
The study - the latest in a series of publications looking at cross-border movements in the context of the Brexit negotiations - reveals differences in the age profiles of the two communities.
More than three quarters (79%) of the people born in the UK and living in Ireland were estimated to be 15 to 64 years old, compared with 55% of those who were born in Ireland and resident in the UK.
Of the Irish-born population living in the UK, two in five (42%) were aged 65 years and over, compared with 10% of the UK-born population in Ireland.
The Office for National Statistics (ONS) report also details how:
:: There are an estimated 110 million border crossings annually between Ireland and Northern Ireland for all reasons including work, business, trade, education, health and family;
:: There were 3.7 million visits from the UK to Ireland last year, with seeing friends or relatives the most common reason for making the trip, followed by holidays;
:: In February 2017, of all UK state pensions received by residents living in the European Union (excluding the UK), 28% were received by those living in Ireland;
:: UK-born people make up one in eight workers in "Culture, media and sports occupations" in Ireland;
:: For Irish-born residents living in the UK, one in three work in a professional occupation with the most common professions teaching and education, nursing and midwifery, and IT and telecommunication.
Emma Rourke, ONS director of Public Policy Analysis, said: "The complex and historic relationship between the UK and Ireland is one of the most challenging areas facing the UK and the EU as Brexit negotiations proceed.
"The sort of analysis we have published today with our colleagues in the Irish and Northern Ireland statistical agencies is going to be indispensable as these negotiations develop.
"It will be more important than ever to have clear and comprehensive data on how and where citizens of our two countries are living and working across borders."