The Treaty of Accession 1972 was the international agreement which provided for the entry of Denmark, Ireland, Norway and the United Kingdom to the European Communities. Norway did not ratify the treaty after it was rejected in a referendum held in September 1972.
Here Jack Lynch, taoiseach/ prime minister, signs for Ireland. Ireland was the poorest country in Western Europe in 1972 with a GDP per capita at 50% of new EEC member Denmark, which was the richest. The 3 new members formally joined on January 1, 1973.
When Ireland joined the European Economic Community in 1973, the Iron Curtain hemmed in millions in the vast prison that was the Soviet Union and its satellite states while military juntas ruled Spain, Portugal and Greece. By any measure, the enlargement to 28 countries and the development of the European Union from the wasteland in the aftermath of World War II, has been a remarkable success of gigantic proportions. Ireland has benefited enormously and compromise is the essence of such a huge project.
Since the UK Brexit vote in June 2016 support in Ireland for European Union membership as reflected in opinion polls has risen to highs of over 90%. However, there has always been a significant transactional aspect to membership including the cute hoorism that has been a perennial factor in domestic politics.