Northern Ireland is part of the United Kingdom, as is England, Scotland and Wales. More precisely, the British state is called the "United Kingdom of Great Britain and Northern Ireland". Northern Ireland is often called Ulster. That is the name of an ancient province. But this province is a little bigger than Northern Ireland.
Northern Ireland is not on the island of Great Britain, the largest of the British Isles, but on the neighboring island of Ireland. The capital is called Belfast. There is a separate government for Northern Ireland, but it only takes care of certain things, especially the police. The rest is done by the British government in London.
Until 1921 all of Ireland was part of Great Britain. Four fifths of the island is in the Republic of Ireland, the remaining fifth is Northern Ireland. Many British people who belonged to a Protestant church live in the northeast of the island. In the rest of Ireland, however, the population is mostly Catholic.
In Northern Ireland there has long been arguments between Protestants and Catholics. There was even violence and many deaths. That has now become much rarer. That is why more and more holidaymakers are visiting the area. You can fish, play golf, go boating or go hiking in the mountains and the rest of the landscape.