This paper will explore theories of mental degeneration in Ireland, and the practical influence of these theories on medical discussion and social policy. The term ‘degeneration’ will be used in this article to define the perceived process of decline, whil e ‘degeneracy’ defines the more general theory of societal, mental, moral, and physical decline. This article will address these ideas, but will not address general eugenic theories, or the relationship between eugenics and the church and state in this era , as these are outside the parameters of this research. The work of British theorists, such as A. F. Tredgold, will be used as a benchmark to define what is meant by the contemporary terms degeneration and amentia . Tredgold notes that ‘amentia’ can be cons idered a lack of mental development, but encompasses numerous levels of mental degeneration ; therefore, amentia was used as an overarching term for idiocy, feeble mindedness, and imbecility (Tredgold, 1920). This paper will also examine contemporary Irish journals to illustrate how these theories were implemented and how medical professionals and politicians perceived mental degeneration to have an effect on the Irish population. Finally, this paper will address the term degeneracy and what this problematic term implied for legislation in the Irish Free State (1921 – 1937).