Welsh poets knew the names of Irish heroes such as Cú Chulainn and Deirdre and Irish narratives influenced the Welsh Mabinogion, yet the degree of Irish influence has been debated for many years. This book is the first comprehensive estimation of the extent to which Irish literature influenced medieval Welsh literature. Sims-Williams's close observation of the physical world, his surmises about human nature in matters of narrative perspective, authorial aim, imagination, and critical reading, and his gentle humor get to the heart of literary creation and sound criticism and are continually informing and refreshing aspects of this book.