Irrational Man helped to introduce existentialism to the English-speaking world and has been identified as one of the most useful books that discuss the subject, but Barrett has also been criticized for endorsing irrationality and for giving a distorted and misleading account of Georg Wilhelm Friedrich Hegel.

He also attempts to explain how the study of science, history, modern art, and religion, specifically Protestantism, highlighted the background growth of existentialism.

"[1] Four existentialist thinkers - Kierkegaard, Nietzsche, Heidegger, and Sartre - receive extended discussion from Barrett, who explains their main ideas and philosophical terminology.

Barrett more briefly discusses other existentialist thinkers such as Karl Jaspers, Nikolai Berdyaev, Martin Buber, Miguel de Unamuno, Gabriel Marcel, Simone de Beauvoir, and Albert Camus, as well as some artists and writers he considers existentialist, such as the Russian novelist Fyodor Dostoyevsky, whom he compares to Nietzsche, crediting him with anticipating Nietzsche's insights into the will to power in Crime and Punishment (1866).

Barrett adds that, "Kierkegaard stated the question of Christianity so nakedly, made it turn so decisively about the individual and his quest for his own eternal happiness, that all religious writers after him seem by comparison to be symbolical, institutional, or metaphorical - in a word, gnostic.

Stewart deemed Barrett guilty of misrepresenting Hegel as a "cosmic rationalist" who, like the philosopher Friedrich Wilhelm Joseph Schelling, and some romantics, believed in a metaphysical world soul.