The 2021 Northern Ireland Census marked the first time since records began where the Catholic share of the population was larger than the combined Protestant share. In 2021, over 42 percent of the population classified themselves as Catholic or from a Catholic background, in comparison with 37 percent classified as Protestant or from a Protestant background. Additionally, the share of the population with no religion (or those who did not answer) was 19 percent; larger than any individual Protestant denomination. This marks a significant shift in demographic and societal trends over the past century, as Protestants outnumbered Catholics by roughly 2:1 when Northern Ireland was established in the 1920s. Given the Catholic community's historic tendency to be in favor of a united Ireland, many look to the changing religious composition of the population when assessing the potential for Irish reunification. Religion's historical influence A major development in the history of British rule in ireland was the Plantation of Ulster in the 1600s, where much of the land in the north (historically the most rebellious region) was seized from Irish Catholics and given to Protestant settlers from Britain (predominantly Scots). This helped establish Protestant dominance in the north, created a large section of the population loyal to the British crown, and saw a distinct Ulster-Scots identity develop over time. In the 1920s, the republican movement won independence for 26 of Ireland's 32 counties, however, the six counties in Ulster with the largest Protestant populations remained part of the UK, as Northern Ireland. Following partition, structural inequalities between Northern Ireland's Protestant and Catholic communities meant that the Protestant population was generally wealthier, better educated, more politically empowered, and had better access to housing, among other advantages. In the 1960s, a civil rights movement then emerged for equal rights and status for both sides of the population, but this quickly turned violent and escalated into a the three-decade long conflict now known as the Troubles.

The Troubles was largely fought between nationalist/republican paramilitaries (mostly Catholic), unionist/loyalist paramilitaries (mostly Protestant), and British security forces (including the police). This is often described as a religious conflict, however it is more accurately described as an ethnic and political conflict, where the Catholic community generally favored Northern Ireland's reunification with the rest of the island, while the Protestant community wished to remain in the UK. Paramilitaries had a large amount of support from their respective communities in the early years of the Troubles, but this waned as the conflict progressed into the 1980s and 1990s. Demographic and societal trends influenced the religious composition of Northern Ireland's population in these decades, as the Catholic community had higher fertility rates than Protestant communities, while the growing secularism has coincided with a decline in those identifying as Protestant - the dip in those identifying as Catholic in the 1970s and 1980s was due to a protest and boycott of the Census. The Troubles came to an end in 1998, and divisions between both sides of the community have drastically fallen, although they have not disappeared completely.

The Good Friday Agreement, or the Belfast Agreement, was the culmination of the Northern Ireland peace process that brought about the end of the 30-year conflict known as the Troubles. The peace process itself involved years of negotiations between the leading nationalist parties (generally seen as representing Northern Ireland's Catholic community), unionist parties (generally seen as representing Northern Ireland's Protestant community), representatives of the British, Irish, and U.S. governments, as well as paramilitary organizations. Terms of the Good Friday Agreement The Agreement involved a series of proposals touching aspects such as demilitarization, identity, devolution, and cross-community cooperation. The Agreement laid the framework for the Northern Ireland government to be reinstated after a 25-year absence, with mechanisms in place to ensure power-sharing and fair representation between Northern Ireland's Catholic and Protestant communities. It also reaffirmed Northern Ireland's position as an equal part of the United Kingdom, but stipulated that the people of Northern Ireland may change this through a referendum (generally assumed to mean reunification with the Republic of Ireland), while it also extended British and Irish citizenship to anybody born in the north. In terms of demilitarization and ending the violence, the Agreement stipulated that all complying paramilitary groups were to disarm and cease operations, and it ensured the British Army presence in Northern Ireland would be significantly reduced. To many, the most controversial aspect of the Agreement was the early release of convicted paramilitary personnel from organization who signed the Agreement, regardless of the charge or length of their remaining sentence - for this reason, the hardline Democratic Unionist Party (DUP) did not support the Agreement. The referendum The Good Friday Agreement was signed by the respective parties on April 10, 1998 (albeit, without the support from the DUP and some smaller paramilitary groups), and was then put to the public on both sides of the Irish border in referenda on May 22, 1998. The vote passed easily on both sides, with ** percent support in the north, and ** percent in the south. In Northern Ireland, the background of voters was not recorded, and therefore there is no official breakdown of the vote by religious background, yet, the most widely accepted estimates suggest that support in Catholic communities may have been as high as ** percent, while support in Protestant communities was roughly ** percent - again, largely in opposition to the early release of paramilitary prisoners.

During the Troubles, the British government introduced a policy of internment in Northern Ireland, whereby authorities had the power to arrest and detain without trial individuals believed to be associated with paramilitary organizations. Between August 9. 1971 and December 5. 1975, almost 2,000 people were interned by the police and British Army. Around 95 percent of these were from a Catholic background, while the first Protestant internees were not arrested until 1973. Internment was introduced in an attempt to weaken the IRA and other nationalist paramilitary groups, however it had the opposite effect. Mismanagement and outdated information saw hundreds of unaffiliated people interned, some of those interned suffered police brutality and were coerced into confessions, and several anti-internment protests turned into some of the bloodiest days of the Troubles. These factors combined to bolster support for Catholic paramilitaries, who gradually transitioned from using defensive to offensive tactics in the early 1970s, and these groups would go on to be responsible for the largest number of combined deaths during the Troubles.

More than 3,500 people were killed as a result of the Troubles in Northern Ireland between 1969 and 1998. Of these, over 1,500 were from Northern Ireland's Catholic community, while just under 1,300 were from the Protestant community. Of the 722 people not from Northern Ireland who were killed in the conflict, over 500 were killed in Northern Ireland, while 120 were killed in Britain, 83 were killed in the Republic of Ireland, and 14 were killed elsewhere in Europe.