In 1920, Ireland became two countries, Northern Ireland and the Irish Republic, both still being under British rule.
The people in Northern Ireland are 2/3 Protestant and 1/3 Catholic, where in the Irish Republic, the people are 95% Catholic. Because of this, in Northern Ireland, the majority Protestants deprived the minority Catholics their political, economic and social rights.
Then after the Catholic minorities unsuccessfully negotiated with the majority Protestants to receive their equal political, economic and social rights, the Catholic Irish Republican Army (IRA) was formed. The IRA is an underground army which uses many types of terrorism and violence to achieve their goals.