Why were many post-war americans concerned about the domestic threat posed by communism? was fear of communism a response to legitimate threats to national security or an irrational response to other tensions within american society? is this applicable today?

They knew that the communists had nuclear weapons as well and there was fear that a nuclear war might broke out after world war 2 had barely ended. There were also a lot of people who supported communism in the United States because they believed communism was more about equality than capitalism was. There was a lot of influential Marxist thinkers who inspired these people.

In the United States it was mostly irrational. At first the United States didn't know how powerful the Soviet Union was because they were closed to the West and people like McCarthy kept talking about traitors who worked in every sphere of life and even in the United State military. People lived in fear not knowing that none of it was actually true and that Soviets were not as powerful as was believed.

It isn't applicable today since nobody really fears communism since the dissolution of the Soviet Union. The 90s led to an era which philosophers called the end of history because it was established that Capitalism won and that the United States was dominant and since then there's been no real reason to fear communism since there was no way of it becoming the main ideology.