This thesis examines the ways in which the Celtic aesthetic emerges in case studies of four writers from the last century: Brian O'Nolan (under the pseudonyms Flann O'Brien and Myles na gCopaleen), David Jones, George Mackay Brown, and John McGahern. It considers a wide selection of their writing across literary genres, including the novel, the short story, the essay, and poetry, but privileges prose and fiction. This study undertakes a formal analysis of these texts using a conceptual, thematic, and critically biographical approach. The archival methodology informing such an approach brings new scholarship into focus that either aligns these authors for the first time or reevaluates their relationships. O'Nolan, Jones, Brown, and McGahern are united here because they put forward their own theories of the Celtic aesthetic and modernized these differing representational strategies when they applied them in their fictional practices. My analysis of each writer begins with a definition of the "Celtic Aesthetic" then draws out how the Celtic is represented in his literary work, showing what we gain from reading the work within a modernist Celtic aesthetic. O'Nolan proposes a Celtic realism within a modernist understanding of the unity between form and content. He writes within a collaborative framework, retrieving modes of thought and literary effects from medieval Irish sources and scholarly texts. He and his peers were concerned with making an Irish-Celtic contribution to modern literature. David Jones develops a visual aesthetic in an Anglo-Welsh context, arguing that the Celtic enhances the potential for metamorphosis through a combination of contraries. Jones establishes a connection between the First World War and ancient Welsh tradition to symbolically pattern the experience of fighting in the trenches. George Mackay Brown shares this idea about Celtic metamorphosis and war. He claims the Celtic is a decorative aesthetic, one that is bound up with Roman Catholic theology and his understanding of Eucharistic anamnesis. Writing almost exclusively about the Orkney islands, Brown portrays the Celtic as an aspect of the Orkney's archipelagic modernism, informed by his own Scottish Gaelic linguistic heritage but also connected by sea to Wales and Ireland. John McGahern implies his theory about Celtic style in his discussions of Gaelic linguistic inheritance and the effect this produces on his English writing. McGahern also shares Brown's mysticism and O'Nolan's practice of depicting eternity in the West of Ireland. There are thus three converging lines of inquiry that will frame this project: first, how does this minor strain in modernist literature animate this set of literary works? Second, how do those characteristics inform our understanding of what the term "Celtic" means in a twentieth-century context and for contemporary readers? And third, what does this contribute to the current field of modernist studies? The Celtic for these writers is transnational, hybrid, decorative, and the means through which their questions about violence and despoliation could find expression in twentieth-century literature.