This thesis explores the nature of the Dindshenchas in the Book of Leinster (s. xii2). The Dindshenchas is a twelfth-century compilation of stories, in prose and verse, which explain the etymologies and origins of medieval Irish place names. The textual history of the Dindshenchas is complex and not yet fully understood, however, the Book of Leinster, as the earliest manuscript, and the only to contain the so-called ‘Metrical Dindshenchas’, is evidently an important witness. Furthermore, the Book of Leinster is codicologically complex, being constituted out of several smaller manuscripts, which are the works of different scribes. In this thesis I explore the nature of the Dindshenchas in this manuscript from a material perspective. An investigation of the codicology reveals that the Dindshenchas was produced by four compilers, independent from one another to varying degrees. Furthermore, these individual collections were themselves compiled over a period of time as poems and items of prose were accumulated. The Dindshenchas in the Book of Leinster, then, is the product of many acts of compilation. This thesis interrogates these acts and their motivations, sitting at the intersection of the material and the conceptual. The physical and visual make-up of the collections – their paratext, mise-en-page, and ordinatio – are used to illuminate the critical categories, interpretations and intellectual frameworks of the compilers. Chapter I considers the nature of dindshenchas as portrayed in the scholarly literature, before situating its etymological kernel within the frameworks of medieval Etymologia and history-writing. Chapter II investigates the codicology of the Dindshenchas in the Book of Leinster in detail, defining the various codicological units which make up the corpus and providing insights into the processes of their compilation. The implications of this research on our understanding of the textual development of the Dindshenchas are profound and will be considered at the end of this chapter. Chapter III discusses the Prose Dindshenchas, exploring the ways in which it was used to structure historical narratives, and its interaction with the wider literary tradition as a text intended for consultation. Finally, Chapter IV turns to the so-called ‘Metrical Dindshenchas’, questioning the motivations behind the individual acts of compilation which produced so diverse a collection and, by extension, the nature of dindshenchas poetry as a meaningful historical category. Cumulatively, this thesis provides greater insight into the Dindshenchas, the Book of Leinster, and the contemporary critical and intellectual environment of the twelfth-century Irish scholars who compiled them.