This thesis examines the transformation of the Gaelic provincial kingdom of Leinster into an Anglo-Norman feudal lordship and explores the role William Marshal, Earl of Pembroke played its subsequent development. There are several strands to this study. It begins with an investigation of the way Marshal –whose role in English history was always relatively well understood by English writers– was perceived by Irish chroniclers and antiquarian scholars writing from the mid-thirteenth century to the 1800s. After this the thesis returns to the origins of the lordship of Leinster where it charts the turbulent rise of the Uí Chennselaigh kings and the campaigns through which they consolidated their hold on the provincial kingdom. While the last of the Uí Chennselaigh kings of Leinster, Diarmaid Mac Murchada, played a well known part in involving Anglo-Norman adventurers in Irish affairs but his motivations and aspirations warrant re-appraisal. The foremost of these adventurers, Strongbow, was not only a formidable military leader but he also set in motion the process of transforming Leinster into a feudal lordship with all the administrative innovations and redistribution of land that this implied. This process was continued by William Marshal who had acquired Leinster by marrying Strongbow’s daughter and heir, Isabella de Clare. Marshal would ultimately rise to the top of Angevian politics but this was only after surviving a series of crises which would engulf the Leinster Lordship. As well as his political and military legacy this study will also look at the physical remains of his time as lord of Leinster; his castles and the religious houses that he founded