The Civil Rights Movements in the southern United States and Northern Ireland were able to mobilize African Americans and Irish Catholics respectively against minority discrimination. These movements initially displayed very similar goals and tactics, looking at courts to counter institutional discrimination, but in successive stages of contention their trajectories fundamentally diverged. While legal mobilization in the United States constituted one of the pillars of the civil rights strategy of contention, in Northern Ireland legal tactics were supplanted by a transgressive (and at times violent) repertoire of contention. To explain this discrepancy, this article relies on the concept of legal opportunity structure (LOS) as an analytical tool to investigate how a state's legal structure affects social movement legal mobilization. Accessibility to courts, availability of justiciable rights and judiciary receptivity are identified as the three core dimensions of the LOS shaping its degree of openness/closure. The paired comparison of these movements reveal that a closed LOS may narrow down the array of tactical options available to social movements, redirecting activists' efforts towards protest. Conversely, an open LOS may encourage legal mobilization as a viable tactical option and, under certain circumstances, even promote contentious activities.