Roger Casement made two journeys to Argentina: the first was in 1907 and the second was in 1910. Little is known about either trip beyond a few fragmentary references in letters and some encrypted entries in the contested Black Diaries. Nonetheless, these traces would suggest that Casement was connected into a vibrant Irish-Argentinian network that played a vital role in the independence struggle before and after 1916. Through reconstructing evidence of these visits and locating his friendships within Ireland’s broader transnational struggle, this essay excavates a dimension of informal diplomacy that prepared the ground for the emergence of Irish foreign policy after 1919.