In the Stone and Bronze Ages, Ireland was inhabited by Picts in the north and a people called the Erainn in the south, the same stock, apparently, as in all the isles before the Anglo-Saxon invasion of Britain. About the 4th century B.C., tall, red-haired Celts arrived from Gaul or Galicia. They subdued and assimilated the inhabitants and established a Gaelic civilization. By the beginning of the Christian Era, Ireland was divided into five kingdoms—Ulster, Connacht, Leinster, Meath, and Munster. Saint Patrick introduced Christianity in 432, and the country developed into a center of Gaelic and Latin learning. Irish monasteries, the equivalent of universities, attracted intellectuals as well as the pious and sent out missionaries to many parts of Europe and, some believe, to North America.

Norse depredations along the coasts, starting in 795, ended in 1014 with Norse defeat at the Battle of Clontarf by forces under Brian Boru. In the 12th century, the pope gave all of Ireland to the English Crown as a papal fief. In 1171, Henry II of England was acknowledged “Lord of Ireland,” but local sectional rule continued for centuries, and English control over the whole island was not reasonably absolute until the 17th century. In the Battle of the Boyne (1690), the Catholic King James II and his French supporters were defeated by the Protestant King William III (of Orange). An era of Protestant political and economic supremacy began.

By the Act of Union (1801), Great Britain and Ireland became the “United Kingdom of Great Britain and Ireland.” A steady decline in the Irish economy followed in the next decades. The population had reached 8.25 million when the great potato famine of 1846–1848 took many lives and drove more than 2 million people to immigrate to North America.

In the meantime, anti-British agitation continued along with demands for Irish home rule. The advent of World War I delayed the institution of home rule and resulted in the Easter Rebellion in Dublin (April 24–29, 1916), in which Irish nationalists unsuccessfully attempted to throw off British rule. Guerrilla warfare against British forces followed proclamation of a republic by the rebels in 1919. The Irish Free State was established as a dominion on Dec. 6, 1922, with six northern counties remaining as part of the United Kingdom. A civil war ensued between those supporting the Anglo-Irish Treaty that established the Irish Free State and those repudiating it because it led to the partitioning of the island. The Irish Republican Army (IRA), led by Eamon de Valera, fought against the partition but lost. De Valera joined the government in 1927 and became prime minister in 1932. In 1937 a new constitution changed the nation's name to Éire. Ireland remained neutral in World War II.

In 1948, De Valera was defeated by John A. Costello, who demanded final independence from Britain. The Republic of Ireland was proclaimed on April 18, 1949, and withdrew from the Commonwealth. From the 1960s onwards, two antagonistic currents dominated Irish politics. One sought to bind the wounds of the rebellion and civil war. The other was the effort of the outlawed Irish Republican Army and more moderate groups to bring Northern Ireland into the republic. The “troubles”—the violence and terrorist acts between Republicans and Unionists in both the Republic of Ireland and Northern Ireland—would plague the island for the remainder of the century and beyond.

Under the First Programme for Economic Expansion (1958–1963), economic protection was dismantled and foreign investment encouraged. This prosperity brought profound social and cultural changes to what had been one of the poorest and least technologically advanced countries in Europe. Ireland joined the European Economic Community (now the EU) in 1973. In the 1990 presidential election, Mary Robinson was elected the republic's first woman president. The election of a candidate with socialist and feminist sympathies was regarded as a watershed in Irish political life, reflecting the changes taking place in Irish society. Irish voters approved the Maastricht Treaty, which paved the way for the establishment of the EU, by a large majority in a referendum held in 1992. In 1993, the Irish and British governments signed a joint peace initiative (the Downing Street Declaration), in which they pledged to seek mutually agreeable political structures in Northern Ireland and between the two islands. A referendum on allowing divorce under certain conditions—hitherto constitutionally forbidden—was narrowly passed in Nov. 1995.

In 1998 hope for a solution to the troubles in Northern Ireland seemed palpable. A landmark settlement, the Good Friday Agreement of April 10, 1998, called for Protestants to share political power with the minority Catholics, and gave the Republic of Ireland a voice in Northern Irish affairs. The resounding commitment to the settlement was demonstrated in a dual referendum on May 22: the North approved the accord by a vote of 71% to 29%, and in the Irish Republic 94% favored it. After numerous stops and starts, the new government in Northern Ireland was formed on Dec. 2, 2000, but it has been suspended four times since then (and has remained suspended since Oct. 2002) primarily because of Sinn Fein's reluctance to disarm its military wing, the IRA, a key stipulation of the Good Friday Accord. Ireland has continued to call for Sinn Fein to comply.

In June 2001, Ireland voted against expansion of the EU to include other countries, which came as a shock to the 14 other EU members. To the relief of the EU, in Oct. 2002 Ireland endorsed the expansion (the Nice Treaty).

Despite a number of recent corruption and bribery scandals, most of which involved the centrist Fianna Fáil party of Prime Minister Bertie Ahern, the party won 81 of 166 seats in May 2002. Ahern became the first Irish prime minister in 33 years to be elected to a second successive term.

In Aug. 2003, Michael McKevitt, leader of the Real IRA, was sentenced to 20 years in prison. The Real IRA split from the IRA because it opposed the Northern Irish peace process.