In 1910, Roger Casement embarked on a voyage into the upper Amazon to officially investigate reports of crimes against humanity committed by a British-financed, Peruvian rubber company. The official report of his findings, published as a parliamentary Blue Book, provoked considerable diplomatic reverberations between Washington, Westminster and Rome. It resulted in a significant shift in international attitudes towards indigenous peoples. The journal kept by Casement during his months in the Amazon demonstrates not merely his own scathing interrogation of the distorting constructs of colonial reality, but a complex recognition of indigenous culture rooted in his own conception of “Irishness.” His defense of “savagery” underscored both his critique of “civilization” and the justification of his tragic revolutionary turn. From 1913, he began to connect the fate of the Amazindian with the lot of Connemara “islanders” suffering from an outbreak of typhus. Later, at his treason trial, his own call to transnational resistance is encoded within the logic: “If there be no right of rebellion against a state of things that no savage tribe would endure without resistance, then I am sure that it is a better thing for men to fight and die without right than to live in such a state of things as this.” This article will reveal the identification with “indigeneity” and the configurations of power evident in the transnational discourse on “indigenous peoples” which was integral to the intellectual formation of anticolonialism in revolutionary Ireland. Through the investigations of Casement, and the establishment of the African Society by the historian Alice Stopford Green, this identification expanded into an explicit recognition of indigenous rights and knowledge and the advocating of a responsibility to defend such rights and knowledge, once independence had been achieved.