Segregation in Northern Ireland is a long-running issue in the political and social history of Northern Ireland. It often been regarded as both a cause and effect of The Troubles between the Roman Catholic and Protestant populations of Northern Ireland.
A combination of political, religious and social differences plus the threat of intercommunal tensions and violence has led to widespread self-segregation of the two communities. Catholics and Protestants lead largely separate lives in a situation that some have dubbed "self-imposed apartheid". The academic John Whyte argued that "the two factors which do most to divide Protestants as a whole from Catholics as a whole are endogamy and separate education".
Soon after they had conquered England, the Normans invaded Ireland. Gaelic lords gradually regained control of the island, until the English Crown's influence was limited to the Pale by the 15th century. The Reformation marked a watershed in the history of the British Isles as the Irish remained Roman Catholic and Henry VIII initiated the Tudor Reconquest of Ireland.
A final part of that century-long struggle was the Nine Years War (1594-1603), after which the defeated earls' lands were confiscated and given to settlers, in the Plantation of Ulster. Most of the planters came from south-west Scotland and became the Scots-Irish. Several others were to follow as attempts to convert the Irish failed, but the plantation of Ulster was to be the largest.
The Plantations had a profound impact on Ireland in several ways. The first was the destruction of the native ruling classes and their replacement by the Protestant Ascendancy, of British (mostly English) Anglican Protestant landowners. Their position was buttressed by the Penal Laws, which denied political and land-owning rights to Catholics and to some extent to Presbyterians. The dominance of this class in Irish life persisted until the late 19th century and cemented the British control over the country.
The following century saw an ever-increasing Irish nationalism, which in turn lead to a resurgence of interest in the Irish language, literature, history, and folklore; by that time Gaelic had died out as a spoken tongue except in isolated rural areas.
Education in Northern Ireland is heavily segregated. Most state schools in Northern Ireland are predominantly Protestant, while the majority of Catholic children attend schools maintained by the Catholic Church. In all, 90 per cent of children in Northern Ireland still go to separate faith schools. The consequence is, as one commentator has put it, that "the overwhelming majority of Ulster's children can go from four to 18 without having a serious conversation with a member of a rival creed."  The prevalence of segregated education has been cited as a major factor in maintaining endogamy (marriage within one's own group) However, the Integrated Education movement has sought to reverse this trend by establishing non-denominational schools such as the Portadown Integrated Primary. Such schools are, however, still the exception to the general trend of segregated education. Integrated schools in Northern Ireland have been established through the voluntary efforts of parents. The churches have not been involved in the development of integrated education.
Historically, employment in the Northern Irish economy was highly segregated, particularly at senior levels of the public sector and in certain sectors of the economy, such as shipbuilding and heavy engineering . Emigration to seek employment was significantly more prevalent among the Catholic population. As a result, Northern Ireland's demography shifted further in favour of Protestants leaving their ascendancy seemingly impregnable by the late 1950s.
A 1987 survey found that 80 per cent of the workforces surveyed were described by respondents as consisting of a majority of one denomination; 20 per cent were overwhelmingly unidenominational, with 95-100 per cent Catholic or Protestant employees. However, large organisations were much less likely to be segregated, and the level of segregation has decreased over the years.
The British government has introduced numerous laws and regulations since the mid-1990s to prohibit discrimination on religious grounds, with the Fair Employment Commission (originally the Fair Employment Agency) exercising statutory powers to investigate allegations of discriminatory practices in Northern Ireland business and organisations. This has had a significant impact on the level of segregation in the workplace; John Whyte concludes that the result is that "segregation at work is one of the least acute forms of segregation in Northern Ireland." 
Public housing is overwhelmingly segregated between the two communities. Intercommunal tensions have forced substantial numbers of people to move from mixed areas into areas inhabited exclusively by one denomination, thus increasing the degree of polarisation and segregation. The extent of self-segregation grew very rapidly with the outbreak of the Troubles. In 1969, 69 per cent of Protestants and 56 per cent of Catholics lived in streets where they were in their own majority; as the result of large-scale flight from mixed areas between 1969 and 1971 following outbreaks of violence, the respective proportions had by 1972 increased to 99 per cent of Protestants and 75 per cent of Catholics. In Belfast, the 1970s were a time of rising residential segregation. It was estimated in 2004 that 92.5% of public housing in Northern Ireland was divided along religious lines, with the figure rising to 98% in Belfast. Self-segregation is a continuing process, despite the Northern Ireland peace process. It was estimated in 2005 that more than 1,400 people a year were being forced to move as a consequence of intimidation.
In response to intercommunal violence, the British Army constructed a number of high walls euphemistically called "peace lines" to separate rival neighbourhoods. These have multiplied over the years and now number forty separate barriers, mostly located in Belfast. Despite the moves towards peace between Northern Ireland's political parties and most of its paramilitary groups, the construction of "peace lines" has actually increased during the ongoing peace process; the number of "peace lines" doubled in the ten years between 1995 and 2005. In 2008 a process was proposed for the removal of the peace walls.
The effective segregation of the two communities significantly affects the usage of local services in "interface areas" where sectarian neighbourhoods adjoin. Surveys in 2005 of 9,000 residents of interface areas found that 75% refused to use the closest facilities because of location, while 82% routinely travelled to "safer" areas to access facilities even if the journey time was longer. 60% refused to shop in areas dominated by the other community, with many fearing ostracism by their own community if they violated an unofficial de facto boycott of their sectarian opposite numbers.
In contrast with both the Republic of Ireland and most parts of Great Britain, where inter-marriage between Protestants and Catholics is common, intermarriage in Northern Ireland is rare. From 1970 through to the 1990s, only 5 per cent of marriages were recorded as crossing community divides. This figure remained largely constant throughout the Troubles, though it has risen to between 8 and 12 per cent according to the Northern Ireland Life and Times Survey in 2003, 2004 and 2005. Younger people are also more likely to be married to someone of a different religion to themselves than older people. However, the data hides considerable regional variation across Northern Ireland.
In the 1970s, the British government took action to legislate against religious discrimination in Northern Ireland. The Fair Employment Act 1976 prohibited discrimination in the workplace on the grounds of religion and established a Fair Employment Agency. This Act was strengthened with a new Fair Employment Act in 1989, which introduced a duty on employers to monitor the religious composition of their workforce, and created the Fair Employment Commission to replace the Fair Employment Agency. The law was extended to cover the provision of goods, facilities and services in 1998 under the Fair Employment and Treatment (Northern Ireland) Order 1998. In 1999, the Commission was merged with the Equal Opportunities Commission, the Commission for Racial Equality and the Northern Ireland Disability Council to become part of the Equality Commission for Northern Ireland.
An Equality Commission review in 2004 of the operation of the anti-discrimination legislation since the 1970s, found that there had been a substantial improvement in the employment profile of Catholics, most marked in the public sector but not confined to it. It said that Catholics were now well represented in managerial, professional and senior administrative posts, although there were some areas of under-representation such as local government and security but that the overall picture was a positive one. Catholics, however, were still more likely than Protestants to be unemployed and there were emerging areas of Protestant under-representation in the public sector, most notably in health and education at many levels including professional and managerial. The report also found that there had been a considerable increase in the numbers of people who work in integrated workplaces.