Catholics currently outnumber Protestants in Northern Ireland, which might be used to strengthen aspirations for a united Ireland.
According to the statistics for 2021, Catholics make up 45.7% of the population, while Protestants make up 43.5%.
A decade ago the census showed Protestants outnumbered Catholics by 48% to 45%, after falling below 50% for the first time. The shift could drive support moves for a united Ireland.
Last year’s figures also showed that 9.3% of the population belonged to no religion, up from 5.6% in 2011.
Northern Ireland was established in 1921 to maintain a pro-British Protestant “unionist” majority as a counterweight to the newly independent and predominantly Catholic Irish state in the south.
At the time the population split was roughly 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 proportion saying they were Irish only was 29.1%, and 19.8% were Northern Irish only.
In the 2011 census, 40% said they were British only, 25% were Irish only and 21% were Northern Irish only.
Colum Eastwood, the leader of the SDLP, the moderate Irish nationalist party, said the shift was “a seminal moment in the history of modern Ireland” that should not be played down.
Sinn Fein MP John Finucane said that “historic change is happening”.
The census also showed a 63.5% increase in the number of people in Northern Ireland with an Irish passport and Brexit is undoubtedly a factor.