From the beginning of the Troubles, groups in Northern Ireland deliberately sought and made use of transnational allies to further their political goals and gain strategic advantages vis-à-vis their opponents. Organizations on both sides of the conflict turned to external allies, including diaspora groups, like-minded movements, and groups with ideological affinities for accessing resources, expanding and practicing their tactical repertoires, and strengthening their claims to legitimacy. While the existence of this transnational dimension of the Troubles is well documented, the differences among cross-border networks—how they were structured, how they functioned, and their impact on the dynamics of the conflict—are less well understood. Drawing on social movement theory, particularly work on transnational advocacy networks, coalition formation, and diffusion, this chapter compares the structure and function of two types of cross-border networks that resulted: licit ties that publicly connected two or more groups, and illicit ties that allowed groups to forge secretive connections with potential allies.