The Demography Of Northern Ireland's First Century

Northern Ireland has becoming more Catholic and less opposed to union with the Republic of Ireland over the last century. 

In part, this is because Catholics had more children than Protestants for a long period of time, although this distinction has slowed to almost nothing, and was counterbalanced before that by emigration from Northern Ireland. 

In part, this is because the Irish Catholic identity has been more stable than the Irish Protestant identity which has trended towards secularism and an Irish identity.

While there are more Catholics than Protestants in Ireland, however, neither is a majority because a significant number of Northern Irish people, 17.4% (disproportionately from Protestant backgrounds), are not religious. About 46% of the non-religious people in Northern Ireland in 2021 were raised that way, while the rest were overwhelmingly raised either Protestant (in most cases) or Catholic (in a much smaller percentage of cases).

In contrast, in the United States, large numbers of people raised both as white Protestants and as white Catholics now identify as having "no religion" consistent with the maxim that a religion that protects a threatened culture thrives relative to a religion that is associated with an establishment culture.

Northern Ireland’s boundaries were drawn up with the explicit aim of creating a two-to-one Protestant majority. At the 1926 census, the first after the war ended, Catholics made up only 35.5% of the province’s population. . . . until relatively recently both groups were largely insulated from other demographic shocks like intermarriage and immigration. (As recently as 2001, Northern Ireland was >99% white, one of the highest proportions in the world; the number today is still very high, at just under 97%.) . . . The Protestant majority was relatively stable for the first forty years or so of Northern Irish history, because a higher Catholic birth rate was offset by higher rates of Catholic emigration; 60% of Northern Irish emigrants before the Troubles were Catholic. However, entering into the 1960s, Catholic emigration fell, and the balance of population began to shift: by 1971, the Catholic population had grown to about 40%. . . . As the Troubles rolled on, Catholic Northerners continued to have more children than their Protestant counterparts. And with a minor baby boom after the peace agreement in 1998, the shift in the province’s demographics has continued apace, especially as the Protestant population has declined in absolute terms. Despite their traditional dominance of Northern Irish politics, since the 2017 election unionist parties (largely supported by Protestants) have been a minority in the Northern Ireland Assembly; and a century after the establishment of Northern Ireland as a Protestant state, the 2021 census showed Catholics outnumbering Protestants for the first time. With Sinn Féin today the largest party in the Assembly . . . many are already suggesting that a united Ireland is now inevitable. . . . While Northern Catholics emigrated much more than Northern Protestants, the disparity is not so stark when we compare them with Southern Catholics, who were also leaving Ireland in droves before the 1960s (at a rate much higher than the small number of Southern Protestants). Northern Catholic emigration was part of a broader history of high levels of Irish Catholic emigration north and south, both before and after the creation of Northern Ireland in 1921; discrimination certainly did not encourage Catholic population growth, but it wasn’t the main driver of emigration. Likewise, the idea that Northern Catholic emigration slowed in the 1960s because of an imposition of welfare and minimal equality from London is hard to square with Southern emigration also declining in the late 1950s and early 1960s. . . . 

The decline in Catholic fertility has tempered the change in demographics: between 2011 and 2021, the Catholic or Catholic background share of the Northern Irish population increased by only 0.6 percentage points (from 45.1% to 45.7%). And indeed, the share of the vote given to Irish nationalist parties in Northern Ireland Assembly elections did not increase significantly over this period (it was just over 41% in 2011, and about 40% in 2022). . . . 

[T]he Alliance Party (a social liberal party which is officially neutral on a united Ireland) more than doubled its seat count in 2022 to become the third-largest party, winning 13.5% of the vote—up from 7.7% in 2011, and only 3.7% back in 2003. This has largely been driven by the affluent and by young people, groups of voters who increasingly feel disconnected from the traditional political and religious categories. Increasingly, people have begun to feel that their political goals and projects can’t be captured by either political tradition, unionism or Irish nationalism. This change is not happening symmetrically, however: the rise of the ‘others’ has eaten much more into the Protestant population than the Catholic one. Unionism is on the decline politically, with unionist parties receiving 40% of the vote in 2022 compared to nearly 47% in 2011, due in no small part to the success of the Alliance Party in affluent Protestant areas. Indeed, Alliance is much stronger east of the River Bann (a traditional dividing line between Catholic-heavy and Protestant-heavy regions), and has struggled with candidate quality in Catholic-heavy areas. And, especially in response to Brexit, more people (especially young people) from a Protestant background are identifying themselves as ‘Irish’ as well as (or instead of) ‘British’ or ‘Northern Irish’. Some of this is being driven by secularisation, which is happening in a lopsided manner in Northern Ireland. Of those who said they had no religion in 2011, a majority identified as British, and over two-thirds identified either as British or Northern Irish; only 14% thought of themselves as Irish compared to 28.4% of the general population, which suggests that those from a Protestant background are secularising more quickly than their Catholic counterparts. Or at least, secularisation in Catholic communities had less of an impact on cultural identity and political beliefs.

From here. A final footnote in the source states:

Note: this essay was written before the crosstabulations were released for the 2021 census; you can find more up-to-date data here.

The top line religion result for Northern Ireland in 2021 is as follows:

Also notably, Irish just became an official language of Northern Ireland once the relevant provision come into force, with the passage of the "Identity and Language (Northern Ireland) Act 2022" on December 6, 2022.