10 May 1972: The Republic of Ireland voted in a referendum to join the European Economic Community. The Third Amendment of the Constitution Act 1972 was an amendment to the Constitution of Ireland that permitted the State to join the European Communities, which would later become the European Union, and provided that European Community law would take precedence over the constitution. It was approved by referendum on 10 May 1972, and signed into law by the President of Ireland Éamon de Valera on 8 June of the same year. Ireland thus joined the EEC along with Britain on 1 January 1973. 

Long mooted by the major political parties [FF & FG] as a solution to the need for  further economic expansion and a reduction on our reliance on the British Economy the State first applied for membership in 1963 when Sean Lemass was An Taoiseach. However our application was tied in with that of the UK and it was rejected as the President of France [Charles de Gaulle] did not want Britain to join  as he felt their presence would seriously weaken France’s influence in the Community. De Gaulle resigned in 1969 and was dead a year later and the way was now clear for both countries to apply again and this was accepted by the EEC States. 

A concerted campaign was undertaken by the Government in the months leading up to the referendum as Minsters made speech after speech to convince the public that such a move was to Ireland’s advantage. The Farming community were assured that farm prices would be far higher with increased sales into the European Markets and the Business Classes that the abolition of tariffs would open up new opportunities for Trade and thus Profits. 

There was some scepticism though especially from the Left with the Labour Party and in general the Republican Movement opposing membership as they felt that the working classes would see very little benefit and such a move would greatly reduce Ireland’s sovereignty and freedom of action on the World stage. 

Probably the greatest factor in its acceptance was An Taoiseach Jack Lynch who used his considerably popularity and perception as a quite thinker to ensure that on the day it was passed by a considerable margin. 

‘The decision which the Irish people will make on 10 May will be recorded either as an unprecedented opportunity which we chose to grasp with incalculable gain, or which we chose to throw away with irreparable loss’

—Taoiseach Jack Lynch, April 1972

https://www.historyireland.com/pushing-an-open-door-ireland-and-the-eec-referendum-of-1972/

In the event the result was 1,041,890 [83.10%] for and 211,89 [16.90%] against. 

The implications were profound as from now on the future destiny of Ireland was inexorably tied in with that of Europe.