Irish and American histories are intertwined as a result of migration, mercantile and economic connections, and diplomatic pressures from governments and nonstate actors. The two fledgling nations were brought together by their shared histories of British colonialism, but America’s growth as an imperial power complicated any natural allegiances that were invoked across the centuries. Since the beginnings of that relationship in 1607 with the arrival of Irish migrants in America (both voluntary and forced) and the building of a transatlantic linen trade, the meaning of “Irish” has fluctuated in America, mirroring changes in both migrant patterns and international politics. The 19th century saw Ireland enter into Anglo-American diplomacy on both sides of the Atlantic, while the 20th century saw Ireland emerge from Britain’s shadow with the establishment of separate diplomatic connections between the United States and Ireland. American recognition of the newly independent Irish Free State was vital for Irish politicians on the world stage; however the Free State’s increasingly isolationist policies during the 1930s to 1950s alienated its American allies. The final decade of the century, however, brought America and Ireland (including both Northern Ireland and the Republic of Ireland) closer than ever before. Throughout their histories, the Irish diasporas—both Protestant and Catholic—in America have played vital roles as pressure groups and fundraisers. The history of American–Irish relations therefore brings together governmental and nonstate organizations and unites political, diplomatic, social, cultural, and economic histories which are still relevant today.