The anti-fascist movement is generally viewed as a reaction to the extreme right by concerned left-wing activists. Therefore we would not expect the Antifa to feature in countries where extreme right activism is not a feature of the politics. However, Ireland has no significant extreme right but it still has an anti-fascist movement that plays an influential role within radical left circles. By treating Ireland as an outlier in relation to the existing reactive explanation for anti-fascist mobilisation this paper takes a deviant case analysis approach to generate novel hypotheses behind the reasons for the anti-fascist movement. First, anti-fascism acts as a site of left convergence, an area of unity that transcends the usual ideological divisions that can impede other types of political collaboration in a fragmented radical left activist base. Second, in the absence of effective extreme right forces, anti-fascism acts as a form of prophylactic action. In effect, the aim of this activism is to deny political space to extreme right micro groups before they become a popular force or a more serious political threat. Finally, a close cultural lineage between elements within the left and a past revolutionary tradition will increase the appeal of anti-fascist activism among left-wing activists.