William Drennan (1754-1820) was one of the earliest architects of Irish Republican thought and a founding member of the Society of United Irishmen. A Presbyterian of Belfast, he rejected the divisions imposed on Ireland and gave enduring expression to the call to “unite Catholic, Protestant and Dissenter” in a common national cause. As a physician to the poor of Belfast, he witnessed at first hand the conditions of the people and the injustice of the existing order. Through his writings and political activity, he called for civil and religious equality and for a fundamental reform of Ireland’s political system. He also played a central role in the establishment of the Belfast Academical Institution, which gave education to people of all backgrounds. In his life and work, Drennan helped lay the intellectual foundations upon which Irish Republicanism would be built.