Cóir Anmann is a late medieval Irish tract made up of numerous 'entries', each of which purports to explain the meaning of a particular epithet associated with a character in early history or mythology. Two separate recensions of this tract have long been known to exist, but in the course of the present study a third recension was identified. One of the main purposes of this thesis is to provide diplomatic editions of these three recensions based on all extant manuscript copies. On the way to producing these editions the manuscript tradition of the tract is reviewed, and a stemma is drawn up for each of the recensions preserved in more than one MS text. English translations have also been made available. In the introductory discussion linguistic and stylistic evidence is used to establish a chronology in which the recensions can be placed relative to one another. To this end verbal systems for the recensions have been prepared. Since the entries in <I>Cóir Anmann </I>are founded on borrowings from pre-existing sources, extracts from independent tracts corresponding to material contained in it have been identified, and suggestions on how these were handled and adapted for use in <I>Cóir Anmann</I> have been advanced. Together with the conclusions reached as to the chronology of the recensions, this study enables the way in which the tract was compiled from disparate sources and 'grew' through the extant recensions to be discerned. The dates of works quoted in <I>Cóir Anmann,</I> internal references and certain aspects of the language are then reviewed to determine the approximate compilation period for each recension. In a final consideration of the development of the tract, this thesis examines histories and genealogies written in the Early Modern period and after for which <I>Cóir Anmann </I>itself served as a source of material.