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The vast majority of the population of Northern Ireland identifies with one of two different groups, unionists and nationalists. Both sides of the community are often described by their predominant religious attachments. Unionists are predominantly Protestant (the major Protestant faith is Presbyterianism, the second in terms of size is the Church of Ireland), while nationalists are predominantly Catholic. However, contrary to widespread belief, not all Catholics necessarily support nationalism, and not all Protestants necessarily support unionism. It is also important to note that, in parallel with other parts of Europe, the proportion of the population practising their religious beliefs has fallen dramatically in recent decades, particularly among Catholics and adherents of mainstream Protestant denominations. This has not necessarily resulted in a weakening of communal feeling.

Political parties in Northern Ireland can be divided into three distinct categories: unionist parties, such as the Democratic Unionist Party, the Ulster Unionist Party, and other smaller parties such as the Progressive Unionist Party and the United Kingdom Unionist Party; nationalist parties, Sinn Féin and the Social Democratic and Labour Party (SDLP); and cross-community parties such as the Alliance Party and the Northern Ireland Women's Coalition.

In a number of areas, the island operates officially as a single entity, for example, in sport. The major religions, the Roman Catholic Church, the Church of Ireland and the Presbyterian Church of Ireland, are organised on an all-island basis. 92% of the population of the Republic of Ireland are Roman Catholic, and 40% in Northern Ireland. Some trades unions are also organised on an all-Irish basis and associated with the Irish Congress of Trades Unions (ICTU) in Dublin, while others in Northern Ireland are affiliated with the Trades Union Congress (TUC) in the United Kingdom. The island also has a shared culture across the divide in many other ways. Traditional Irish music, for example, though showing some variance in all geographical areas, is broadly speaking the same on both sides of the border. Irish and Scottish traditional music are similar.

The Anglo-Irish Treaty was narrowly ratified by the Dáil in December 1921 but was rejected a large minority, resulting in the Irish Civil War which lasted until 1923. In 1922, in the middle of this war, the Irish Free State came into being. For its first years the new state was governed by the victors of the Civil War. However in the 1930s Fianna Fáil, the party of the opponents of the treaty, were elected into government. The party introduced a new constitution in 1937 which renamed the state to simply "Ireland".

From its creation in 1921 until 1972 Northern Ireland enjoyed limited self-government within the United Kingdom, with its own parliament and prime minister. However the Protestant and Catholic communities in Northern Ireland each voted almost entirely along sectarian lines, meaning that the government of Northern Ireland was dominated by the Unionist majority which did not permit Catholics to participate in the government.