This thesis examines the contrasting ways in which the literary works of William Butler Yeats, James Joyce, and Samuel Becket engage with Friedrich Nietzsche’s philosophy as it is understood today. Using late twentieth-century and contemporary interpretations of Nietzsche’s philosophy as its primary methodological framework, the study identifies new parallels and dissimilarities between these Irish modernists. As this philosophy was interpreted in a myriad of complex and often contradictory ways, the first chapter adopts a history of ideas approach to establish the many diverse ways that Nietzsche’s work was read throughout the decades in which Yeats, Joyce, and Beckett lived and worked. Each of the remaining chapters focus on one of four central themes in Nietzsche’s writing: eternal recurrence, the Übermensch, transnationalism, and ethics. These thematic chapters are comparative and explore the degree to which Yeats, Joyce, and Beckett interact with these themes as they are understood in the Nietzschean lexicon. The chapters are arranged in an order that captures the trajectory of Nietzsche’s philosophy when it is considered as a whole. The theory of eternal recurrence is addressed first because, for Nietzsche, it functions as a litmus test for the mode of existential authenticity personified by the Übermensch. Accordingly, the Übermensch is considered in the chapter that follows. The two remaining chapters focus on that which must be overcome for this mode of authenticity to be realised, namely the nation state and the ethical values that underpin its customs and laws. This structure also shows that Nietzsche’s writing operated as a lightning rod for aesthetic modernism and that the work of Yeats, Joyce, and Beckett charts the myriad directions in which its currents flowed. There is already some critical consensus regarding the specificity and extent of these Irish modernists’ engagement with Nietzsche’s ideas. Yeats, who repeatedly acknowledged his admiration for Nietzsche, is seen as the most ‘Nietzschean’ of the three because his work dramatizes key Nietzschean motifs in fairly direct and vivid ways. In the case of Joyce, scholars have more recently begun to reconsider the long-popular notion that Joyce identified with Nietzsche’s self-created individual in his youth, before outgrowing these ideas as he matured. Beckett is regarded as the least ‘Nietzschean’ of the three and is believed to have expressed little or no interest in Nietzsche’s philosophy. Although this analysis of the Yeats/Nietzsche relationship is not without merit, this comparative study demonstrates that Beckett’s work, and to a slightly lesser extent Joyce’s work, is in many important ways more ‘Nietzschean’ than Yeats’s literary output.