The conflict between England and Ireland stems back to the Norman conquest of Ireland in the 12th century, and the claiming of overlordship of Ireland by the King of England. Ireland had its own High King (the last of whom was murdered on the orders of the King of England), its own Celtic culture, language and history. It was gradually colonized by the English who increasingly asserted their control over the island and eventually reduced its inhabitants to the status of aborigines, treating them appallingly and trying to stamp out Irish religion, laws, cultures and traditions, eventually incorporating Ireland into what became known as the United Kingdom of Great Britain and Ireland. This process went on for centuries with periodic rebellions of the native Irish which met with various successes and failures, culminating in the Rising of 1916 which was pivotal in turning ordinary Irish people against being part of a union with Britain, and led to a war of independence which ended with an agreement by which Ireland would be given quasi Independence, as long as part of Ulster was partitioned away to remain within the Union with Britain (Ulster had been colonized by Ulster Scots Protestants in Tudor times), this led to the formation of a 26 county Irish Free State and a self-ruling 6 county Province of Northern Ireland in 1923.

The Free State declared itself a Republic in 1948 and has remained so ever since. The history of Northern Ireland was complex and troubled, it’s Protestant majority treated the Provinces’ Irish Catholic minority very poorly culminating in the civil rights protests of the late 1960s, which were brutally cracked down on by the largely Protestant security forces, this led to a conflictual situation in which the IRA or Irish Republican Army, became a defender of Catholic’s against what they saw as Protestant domination. This became so bad and had become a de facto civil war, that the Government in Westminster realized it would have to send in the British Army in an attempt to keep the peace, and ultimately took Governance of the Province away from the Parliament in Belfast and transferred power back to Westminster.

However, a situation soon developed in which Catholics perceived the British Army to be on the side of the Protestant majority and an occupying force in Catholic areas. A split in the IRA occurred with the formation of a more militant and violent Provisional IRA being formed, which carried out a vicious war against the British Army and Northern Protestants, who had formed their own paramilitary organizations such as the UDA and UDF. The seventies became a decade of terror on the streets of Northern Ireland, sometimes spilling over into Britain and the Republic of Ireland.

This civil conflict continued until by the early 1990s, war weariness on all sides led to the development of a peace process by which the British agreed to return the Governance of Northern Ireland back to a parliament in Belfast which through a complicated system would involve a system of power-sharing between both Protestants and Catholics. The IRA declared a Ceasefire in 1994, Protestant paramilitary groups did likewise, and the British started to demilitarize the Province. It’s been a bumpy road, but the peace has held until the present day, and power-sharing governance remains in Northern Ireland. Incidentally, Northern Ireland voted to remain within the EU during the Brexit vote, like Scotland did.

That’s a very potted history of the centuries long strife between England (Britain) and Ireland and how the island was partitioned and the troubles and peace process occurred in Northern Ireland. There are a great many excellent history books on this subject and I suggest you google one of them and purchase it as it’s a fascinating topic.