It was introduced and signed into law on 2 September 1939, the day after the Invasion of Poland by Germany and allowed the government to exercise emergency powers during World War II (known in Ireland as The Emergency) although the state was neutral.
The First Amendment specified that "time of war" could include an armed conflict in which the state was not actually taking part.
The amendment was introduced by the Fianna Fáil government of Éamon de Valera on 2 September 1939, and passed swiftly through both houses of the Oireachtas.
The Second Amendment, passed in 1941, also under Article 51, clarified that emergency provisions must be within the time of war or armed rebellion itself and added a clause at the end of the last sentence, which specified that a "time of war" could extend beyond the termination of hostilities.
Those later changes are highlighted in bold: First Amendment of the Constitution Bill 1939 on the Oireachtas Beta website