Clonmacnois was one of the main ecclesiastical centres in early Christian Ireland. Yet no comprehensive work has hitherto been published which examines its history as an institution of religious, social and economic life. This book undertakes a detailed analysis of Clonmacnois before and during the age of reform and assesses possible reasons for its subsequent decline as an ecclesiastical centre. It traces the history of the former lay-ecclesiastical aristocracy down to the later Middle Ages, and, using previously neglected evidence surviving in seventeenth-century transcripts, sets out to reconstruct the extent of the former monastic lands.