The book is an attempt to interpret the dramatic changes that affected both the social structure and the culture and language of Ireland during the 9th-12th centuries. These changes began with the arrival of the Vikings in Ireland, and the consequences of this almost three-century-long conquest are usually viewed negatively. However, the author tries to look at this long-standing problem in a new way and seeks to find the positive aspects of the presence of the Vikings in the social and cultural space of Ireland. On the one hand, he sees them in numerous acquisitions (shipping, fishing, the emergence of cities, the emergence of markets, coinage, etc.), on the other hand, he describes the consequences of the destruction of the isolation of the Irish culture of the early Middle Ages, manifested in the emergence of new themes and plots in the narrative tradition, the emergence of their own historiography, openness to the influence of European culture in general, including ancient. Social and cultural changes inevitably led to purely linguistic changes, which are explained by the influence of the Vikings only indirectly, but which also form a coherent system.The publication is intended for all those interested in the Celtic and Scandinavian Middle Ages, as well as for philologists, historians and general linguists.