There are now more Catholics living in Northern Ireland than there are Protestants and could be used to support calls for a united Ireland.
Figures for 2021 show that 45.7% of the population is Catholic, compared with 43.5% Protestant.
A decade ago, the census showed that Protestants outnumber Catholics by 48% to 45%, after falling below 50% for the first time. This change could spur moves in favor of a united Ireland.
Last year’s figures also showed 9.3% of the population had no religion, up from 5.6% in 2011.
Northern Ireland was formed in 1921 to maintain a pro-British Protestant “trade unionist” majority as a counterweight to the newly independent and predominantly Catholic state of Ireland to the south.
At that time, the population was about two-thirds Protestant and one-third Catholic.
The census also asked about people’s sense of national identity, with 31.9% saying they were British only and 8% saying they were British and Northern Irish.
The percentage who said they were only Irish was 29.1% and 19.8% were Northern Irish only.
In the 2011 census, 40% said they were only British, 25% only Irish and 21% Northern Irish.
Colum Eastwood, leader of SDLP, Ireland’s moderate nationalist party, said the change was “a pivotal moment in modern Ireland’s history” that should not have happened.
Congressman John Finucane of Sinn Fein said “historical change is underway”.
The census also shows a 63.5% increase in the number of people in Northern Ireland holding an Irish passport, and Brexit is certainly a factor.