James Connolly is by most measures a canonical figure. He is widely recognized as a founder of the national and labor movements in Ireland: an organizer, agitator, journalist, trade union leader, and, ultimately, republican martyr. Yet his status as a political thinker has been curiously diminished and overlooked. Seeking to rectify this neglect, I undertake in this article a historical reconstruction of Connolly’s political thought, demonstrating that he was, contrary to the anti-intellectual anachronism of leading Irish historians, a complicated and innovative republican theorist. In the first section, I draw out Connolly’s understanding of colonialism and its relationship to both historical development and revolutionary subjectivity, pointing to the universalist and teleological dimensions of his thought, as well its boundaries. Second, I focus on Connolly’s fraught but productive encounter with Irish nationalism and the cultural flourishing it drove, reconstructing his strenuous efforts to cast the Irish nation and the proletariat as synonymous. Third, I probe the conceptual substance of his socialist republicanism, elucidating the centrality of two republican doctrines—popular sovereignty and freedom as nondomination—to Connolly’s thinking. Last but not least, I map the political and intellectual antecedence of Connolly’s decision to partake in the Easter Rising of 1916, his final act.