The existing literature views the anti-fascist movement as a reaction to far-right activism by concerned left-wing activists. Therefore we would not expect anti-fascism to feature in countries where far-right activism is not a feature of the politics. However, there are places where anti-fascism has become a significant feature of radical left politics even where no extreme right movement exists. This paper aims to generate hypotheses about the anti-fascist movement from a case study of one such country. Ireland has no coherent far-right yet still has an anti-fascist movement that plays an influential role within radical left circles. By placing Ireland in a comparative context and through the use of interview data from anti-fascist activists; this paper will consider three possible causal factors in the development of the anti-fascist movement that goes beyond a simple left-wing reaction to fascism. 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 far-right forces, anti-fascism acts as a form of prophylactic action. In effect, the aim of this activism is to deny political space to far-right micro groups before they become a popular force or a more serious political threat. And 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; even in the absence of a far-right threat. So, politically motivated violence against the far-right becomes a viable option at an earlier point, if anti-fascist activists embrace a tradition that views militant resistance to oppressive forces as being a valid and necessary option.