This thesis focuses critical attention on the long-standing claim that the Irish population was fully literate in the twentieth century. Unquestioned assertions that marginalize the illiterate Irish person are supported by a limited set of documents. This claim is revisited using a wider range of written materials, located in publicly-accessible archives. A thematic analysis moves beyond the surface semantic level of the data to explore the shared assumptions, conceptualizations, and discursive resources that contribute to the social construction of literacy, illiteracy, and the illiterate person. The thesis adopts the position that literacy and society are entwined in a complex and dynamic relationship. It explores one dimension of this relationship by asking: How does Irish society construct the illiterate person? A chronological approach spanning the years 1900-1980 describes how several discourses of literacy operate to produce different constructions of the illiterate person. The documentary evidence provides access to a range of shared discursive resources and their influence on material conditions for a significant minority in twentieth-century Ireland. Three key findings are presented. One is the presence of the illiterate person within mainstream Irish society, in contrast to prevailing accounts that locates those with literacy difficulty at the margins. A second key finding is that a continuum of literacy is evident in the data. Stratified forms of literacy, a hierarchy of readers, and multiple subject positions for the illiterate person provide alternative ways to conceptualise literacy proficiency, moving beyond a simple dichotomy of literate and illiterate. The third key finding is that silences in relation to literacy in these documents are not innocent omissions, but instead provide strategic support for claims to full literacy. The study ultimately produces a challenge to existing accounts that reify literacy proficiency as a key distinguishing feature of the Irish nation-state.