The idea that mass migration from nineteenth-century Ireland created an Irish 'empire' has had enduring appeal. Yet as many historians of Ireland, its diaspora and particularly the Irish Catholic Church have noted, the existence of a peculiarly Irish 'spiritual empire' was widely spoken of even as the country's ports remained choked with emigrants. A merging of Irish migration and religious history demands a more detailed and focused treatment of what was a long-running and widespread facet of the clerical discussion of emigration. This chapter looks at the set of ideas that lay behind the concept of a special emigrants' mission. It then traces the development of its expression and any challenges to it, including parallel evocations of the concept from Irish Protestant clergy. The chapter examines some important practical consequences of emigration and the 'spiritual empire' for the Irish churches.