When ideas or tactics catch on across parties in a multiparty system the process is termed contagion. Scholars have identified many examples of contagion, particularly dealing with gender quotas and the number of female candidates. The theory of contagion has, however, suffered from both narrow application and underdevelopment historically, particularly because the literature on contagion focuses exclusively on relations between parties. Yet the Northern Ireland Women's Coalition (NIWC), a tiny short-lived political party dedicated to ‘equitable and effective political participation’ for women, indicates that structural, historical and even international circumstances can facilitate or limit this process of transfer. Using the case study of the NIWC, which improved women's representation in nearly all the other political parties in the system, this article identifies six key variables that condition contagion. In addition, the article traces the NIWC's effects on multiple dimensions of women's representation, descriptive and substantive, in other parties. Considering the process of contagion more broadly enhances the theory but also may provide practical guidelines for expanding policy ideas related to women's representation, human or civil rights protections or environmental initiatives across political parties and across political systems.