500 BC Celts arrived and Gaelic became the language of Ireland
400-500 AD Christianity overtook the indigenous pagan religion
800-900 Viking raids 
1014 Brian Boru, the High King of Ireland, defeated vikings.
1100s  Normans arrived building walled towns
1534 King Henry VIII declared King of Ireland 
1541-1600s English plantation policy led to English and Scottish Protestants settling in Ireland causing sectarian conflict.
1600s Penal laws disempowered Catholics
1601 Spanish failed in Kinsale in attempt to free Ireland from the British. 
1791 United Irishmen formed to bring Irish people together to reduce Britain’s power in Ireland. This rebellion failed.
1801 Act of Union was passed uniting Ireland politically with Britain.
1829 Act of Catholic Emancipation passed in London. 
1845-1847 blight struck potato crops nationwide but Ireland was forced to export harvests of wheat and dairy products to Britain.
1845-1851 two million people died or emigrated from Ireland. 
1878 Irish Home Rule Party challenged Britain’s control of Ireland. In Ulster, N. Ireland, Protestants concerned about Home Rule preferred union with Britain. They threatened an armed struggle for a separate Northern Ireland if independence was granted to Ireland.
1912  Home Rule Bill was passed but was not brought into law. 
1914 The Home Rule Act was suspended at the outbreak of World War One. 
April 24-30 1916  Easter Rising The Irish Volunteers and the Irish Citizen Army read the Proclamation of the Republic in Dublin declaring an Irish Republic independent of Britain. Battles ensued with casualties on both sides.The Easter Rising ended with the surrender of the rebels. The British unwittingly created martyrs by executing many of the leaders and participants . 
January 1919 House of Commons gathered in Dublin to form an Irish Republic parliament with power over the entire island.
1919-1921 The ‘war of independence’  the Irish Republican Army waged a guerilla war against British forces. Sinn Fein, means "Ourselves Alone" became a political movement for freedom. At war's end the Irish Free State was formed. Ireland was divided into the Republic of Ireland, which is an independent country, and Northern Ireland, which is still a part of the United Kingdom.
1921 a treaty signed by the Irish and British authorities granting independence to Ireland split Irish public and political opinion.
1922-1923  Civil War followed between pro and anti treaty forces. The consequences of the Civil war can be seen to this day where the two largest political parties in Ireland have their roots in the opposing sides of the civil war – Fine Gael (pro-treaty) and Fianna Fáil (anti-treaty). A period of political stability followed the Civil War.
1960s Discrimination against Catholics causes unrest.
1968 Catholic civil rights marches in Northern Ireland led to violent reactions from Protestant loyalists and the police. 
1969 British troops were sent to Derry and Belfast to maintain order and protect the Catholic minority.
1969-1998 'The Troubles’ begin in N. Ireland when nationalist/republican and loyalist/unionist groups clashed violently.
1972 Bloody Sunday, British forces opened fire on a Catholic civil rights march in Derry killing 13 people. 
1973 Ireland joined the European Economic Community (now the European Union).
1998 The Belfast Agreement was signed putting an end to The Troubles.
1980s Irish economy fell into recession causing many to emigrate.
1990s Economic reforms of the 80s created one of the world’s highest economic growth rates. Ireland became a country of immigration with a period called the Celtic Tiger.
2020 Britain left the European Union causing concern over the relationship between N. Ireland and the Republic of Ireland’s border vs the potential of becoming one nation again.

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Generally speaking, The Republic of Ireland is native Irish and has a Catholic majority (Loyalists) while Northern Ireland is comprised of mostly protestant citizens descended from Great Britain (Unionist) and therefore loyal to Great Britain.