Edward Baker Littlehales arrived in Ireland in 1798, beginning a career which would not see him depart until 1819. His tenure would see him witness and act in a variety of significant historical events in Ireland, including the 1798 rebellion, the passing of the Act of Union, Emmet's rebellion, as well as the recurrent threat of invasion from France. From 1801 to 1819 he worked in Dublin Castle as military under-secretary, one ofthe more senior members of the administration. Despite this he remains an understudied figure in Irish history. Drawing from primary sources, including Littlehales' personal papers, military and governmental records, and contemporary correspondence, this study reconstructs Littlehales' career trajectory as under-secretary and his contributions to military governance and administration between 1801 and his retirement in 1819. The research situates Littlehales within the broader context oflrish military and administrative history, highlighting the centrality of his role in shaping military policy in post-union Ireland. Littlehales' length of service offers a unique prism through which to view the attempts to implement and normalise the Act of Union. In conclusion, Edward Baker Littlehales emerges from this research as a key architect of post-union administration in Ireland. This dissertation not only recovers the significance of his contributions but also provides a framework for analysing the evolution of administrative practices in the context of broader historical developments. Importantly, the critical role of personality in these administrative practices is given due consideration.