Hippolyte Delehaye noted that the date and place of a saint's death, both usually recorded in martyrologies, are the two essential items of any saint's dossier. However, it has been a feature of recent scholarship of medieval Irish hagiography that saints' vitae have proved far more attractive as objects of study. With their fantastical stories and political biases, vitae are perhaps more alluring – though usually also less reliable – than the sparse and sometimes impenetrable martyrologies. It is to be hoped that the publication of Pádraig Ó Riain's book might inspire scholars to undertake the daunting task of producing editions of the extant martyrological texts. Some medieval Irish martyrologies have been edited, but most of these editions are more than a century old and do not conform to modern scholarly standards. Other texts are yet to be edited at all. Ó Riain has been one of the few scholars in recent years to tackle these important sources, and his fascinating book combines significant new research with updated versions of articles that have been published previously in journals and as part of lecture series. The bulk of the book deals with the Martyrology of Tallaght, its textual history, sources and relationship to the so-called Hiernonymian martyrology. There are also chapters on the Martyrology of Óengus, the Martyrology of the Regensburg Schottenkloster (including an edition), the Martyrology of Donegal and a number of lesser-known texts, some of which Ó Riain recently edited for the Henry Bradshaw Society under the title Four Irish martyrologies. Feastdays of the saints is a book that will benefit researchers in all areas of medieval hagiography: of particular note are the copious indices (including an index of manuscripts and an index sanctorum) and appendices (including a chronology of redactions and various stemmata), which make this book an extremely useful research tool. However, this reviewer cannot help but wonder whether, to some extent, the cart is being put before the horse. Ó Riain states in his introduction that the aim of his volume is ‘that, guided by my conclusions, scholars will in future be able to use the texts of Irish martyrologies – including, wherever present, prefaces, commentaries and glosses – with some confidence in the dates to be assigned to these documents, and with some knowledge of their place in the textual history of the tradition to which they belong’. However, without access to accurate critical editions of the key texts, it will be difficult for scholars actively to engage with Ó Riain's arguments regarding the textual history and importance of the Irish martyrological tradition.