The late sixteenth-century Book of Ballycummin (RIA MS 967/ 23 N 10) is a rare example of a composite vellum and paper Irish manuscript. Yet the original arrangement of the quires and the sequence of the texts within it is unclear since the manuscript was unbound and rebound several times in its later history. Without understanding the original structure of the manuscript, it is not possible to read the texts in their proper manuscript context or to situate the codex within the broader context of the Irish manuscript tradition. Therefore, this chapter has two aims. Firstly, it examines the codicology of the book to establish the original structure and sequence of quires and place the texts in their original order. Secondly, it examines the use of filler-texts, many of which have been overlooked by scholarship, together with their textual history, and assesses their purpose in the codex. It is argued that such filler-texts were not only intentionally selected for their content, but also hold a particular function for the cohesion of the book.