This dissertation explores the *specula principum*, or advice treatises for rulers, written in Ireland and the Carolingian world between the seventh and tenth centuries. Including collections of aphorisms, the deathbed speeches of literary figures, formal manuals on courtly behavior and other texts in Old Irish and Latin, these advice manuals offer insight into several aspects of early medieval European culture. This work contains two major arguments. First, I argue that the intense interest in this genre across northwestern Europe suggests that the concept of what made a good leader was changing, which I show to be the result of a negotiation between an older, pre-Christian moral code and the emerging Christian concept of morality. Second, I demonstrate the interconnectivity of these regions by making a case for Irish influence on the Carolingian manuals, which most likely occurred when Irish missionaries and scholars brought ideas to the continent in the eighth and ninth centuries. This dissertation is divided into four parts. The first addresses the Old Irish and Hiberno-Latin materials, beginning with a discussion of the central concept of Irish kingship, the *fír flathemon*. Turning to the *specula* texts themselves, I organize the advice for kings into three categories based on its range of influence: universal, public, and personal. Also addressed is the Christian influence on these mostly traditional texts, as well as the changing nature of idealized masculinity revealed within the advice on leadership and personal behavior. The following chapter parallels the first but addresses the Latin *specula* written for a Carolingian audience with a focus on the influence of secular sanctity. This allows for comparison of the two bodies of texts while also highlighting the Irish influences on the Carolingian material. Section three is focused on the portrayal of women within the various regions, making the argument for the increasing importance of the Christian family unit as a microcosm for the larger kingdom. The final section addresses the implications of the *speculum principis* genre for the discussion of early medieval king-making ceremonies, showing where themes from the *specula* have found their way into the coronation oaths and evaluating the evidence for community participation in the ceremonies.