The O'Shea trial and the ensuing fall of Charles Stewart Parnell occupy an epochal position in accounts of the sexual politics of Victorian Britain and the development of Irish nationalism. This article examines how the 'Parnell myth' came to serve (and was constructed from the outset) as a symbolic edifice within which anxieties concerning the relationship between Irishness and sexuality could be foregrounded and negotiated. In particular, it will analyse Timothy Healy's influential post-split denunciations of Parnell, and the rhetoric of sexual contagion through which they were conducted, a campaign which set the discursive terms of twentieth-century mainstream Irish nationalism. Through an analysis of Healy's post-split journalism, contemporary political memoirs by T.P. O'Connor, and a range of nineteenth-century medical and psychiatric texts, this article will highlight the ways in which discourses of sexual health were used to reshape Parnell's public persona at the level of gender and ethnonational affiliation. In doing so, it will illustrate how a sensitivity to the history of medicine can enrich critical understandings of a crucial moment in the political and cultural history of Ireland, and shed fresh light on the vexed collocation of Irish identity and sexual purity which the Parnell split reinforced.