This article assesses ‘Rotten Protestants’, or Protestant home rulers in Ulster, by means of an analysis of the Ulster Liberal Association, from its founding in 1906 until its virtual disappearance by 1918. It argues that Ulster Liberalism has been neglected or dismissed in Irish historiography, and that this predominantly Protestant, pro-home rule organization, with its origins in nineteenth-century radicalism, complicates our understanding of the era. It has previously been argued that this tradition did not really exist: this article uses prosopography to demonstrate the existence of a significant group of Protestant Liberal activists in Ulster, as well as to uncover their social, denominational, and geographic profile. Ulster Liberals endured attacks and boycotting; this article highlights the impact of this inter-communal violence on this group. Although Ulster Liberalism had a substantial grassroots organization, it went into sharp decline after 1912. This article describes how the third home rule crisis, the outbreak of the Great War, and the Easter Rising of 1916 prompted a hardening of attitudes which proved detrimental to the survival of a politically dissenting tradition within Ulster Protestantism.