The state composed of the southern 26 of Ireland's 32 counties has had three different names, which reflect the stages by which the goals of the defeated antitreaty side were actually attained during the generation after the civil war.
Under the 1922 constitution framed by the protreaty side, the first prime minister was William T. Cosgrave (1880-1965). De Valera's republican party refused to sit in the Dail because of the required oath of allegiance to the British crown. Despite the difficulties of governing a state whose very legitimacy was rejected by the major opposition party, the Cosgrave government managed to set up a well-functioning administration and to accomplish some modest reforms. In 1927, however, de Valera reconciled his conscience to taking the oath while denying that he was doing any such thing. After the 1932 election his Fianna Fail party (with Labour party support) was able to form a government, and, as prime minister until 1948 and again in 1951-54 and 1957-59, de Valera consolidated his party's dominance over Cosgrave's party, Fine Gael.
In 1937 a new constitution drafted by de Valera was adopted. The new state, Eire, a republic in all but name, remained formally within the British Commonwealth. During World War II, supported by the majority of the southern population, de Valera followed a policy of neutrality.
In 1948, John A. Costello (1891-1976), a Fine Gael leader who succeeded de Valera as prime minister in a coalition government, introduced legislation by which the South became a republic outside the Commonwealth. In the 1950s the republic began to turn away from constitutional struggles and towards a greater concern with economic development. The attempt to achieve economic self-sufficiency, a prominent feature of the 1930s, gave way to policies of interdependence.
Under Fianna Fail prime minister Sean Lemass (1899-1971; served 1959-66), the republic entered into a free-trade agreement with Britain. His successor, John Lynch, led (1973) the country into the European Economic Community. Lynch was displaced by Fine Gael's Liam Cosgrave in 1973. Fianna Fail returned to power from 1977 to 1981, led first by Lynch and, from 1979, by Charles J. Haughey. Fine Gael leader Garret FitzGerald led a coalition government from 1982 to 1987, when Haughey again took over, continuing in office until 1992. Albert Reynolds of Fianna Fail (1993-94) was succeeded by Fine Gael's John Bruton.
Feudalism had been the essential ingredient of state building that Ireland lacked in the Middle Ages; nationalism may be the essential prerequisite for successful state building in a democratic age. The nationalism that had been growing among Catholics for a century was a prime factor that enabled the South to overcome the bitterness of civil war.