Irish nationalists were themselves riven by civil war after the treaty and the partitioning, though the 26 counties later became fully independent in the late 1940s as the Republic of Ireland.
In Northern Ireland, differences between nationalists who backed a unified Ireland and unionists who supported the United Kingdom split strongly along religious lines, and Protestants tended to occupy a place of social and economic privilege.
In the 1960s, Catholics began to push for civil rights, voting rights, police reform, and an end to discrimination. Tensions turned violent in 1968, after civil rights demonstrators faced violent opposition from their opponents and police inaction.
The subsequent period known as The Troubles featured riots, violent attacks, bombings and retaliation from Protestant and Catholic paramilitary groups, as well as involvement from the Royal Ulster Constabulary police, intervention from the British military, and mass internment of civilians.
The Good Friday Agreement of April 10, 1998 largely brokered peace on the island, but significant tensions over past crimes remain.
New unrest in Northern Ireland began in late March of this year, taking the form mainly of young people throwing bricks, fireworks, and other projectiles at police.
According to the BBC, that unrest largely erupted as a result of police in March choosing not to prosecute members of the left-leaning Sinn Fein party for violating coronavirus restrictions last year, as well as continued tensions over a new sea border between Northern Ireland and Ireland imposed as a result of Brexit.
In May this year, a new inquiry ruled that a Catholic priest, a mother of eight, and at least seven other civilians were wrongly killed by British soldiers in Northern Ireland in a three-day 1971 incident known as the Ballymurphy Massacre. The official finding places the incident as a possible forerunner to Bloody Sunday, another massacre of Catholic demonstrators by British paratroopers.