In 1921, the island was divided into Ireland and Northern Ireland.

An excerpt from the article 39 facts about Ireland

Northern Ireland is part of the United Kingdom of Great Britain and Northern Ireland, occupies the northeastern part of the island of Ireland, and its capital and largest city is Belfast.

Conflicts in Northern Ireland have been ongoing since the Middle Ages when all of Ireland came under the rule of the Kingdom of England. Already in the 17th century, the settlement of Ireland by the English and Scottish people began by removing the Irish. This caused hostility between the English and the Irish, compounded by religious differences - the Irish were Catholic and the English were Protestant. When independent Ireland was established in 1921, Northern Ireland remained within Great Britain and was mostly inhabited by Protestants of English origin.