At the intersection of economic and political history, this book explores the pivotal role of the Irish diaspora in the development of the Republic of Ireland and Northern Ireland. It examines the strategies their respective authorities used to harness it. While post-partition efforts were limited, from the 1950s onward both Irelands successfully engaged diaspora business elites to attract investment and stimulate growth. Highlighting the discreet yet decisive influence of powerful Irish-American businessmen, the study analyses their complex interactions with Irish policymakers. Marked by mutual interests, strategic collaboration, and subtle power dynamics, these relationships played a crucial role at key moments in Ireland's economic history. Drawing on a century of previously unpublished archival sources from Ireland, Northern Ireland, and the United States, including government records, parliamentary papers, and contemporary press, this book provides fresh comparative insights into the economic policies of the two Irelands and the enduring impact of the diaspora on Ireland-US relations. Offering a rigorous and engaging analysis, this book demonstrates how the entrepreneurial networks of the Irish diaspora have been instrumental in shaping modern Ireland. It is essential reading for scholars in economic and business history, Irish history, and diaspora studies.