Paul Cullen became Ireland’s first cardinal in 1866, an appointment which formalised his long-standing influence in the Propaganda Fide, especially in the appointment of bishops in the English-speaking world. This chapter considers two such interventions, in Scotland and Australia, where Cullen sought to use his influence, unsucessfully in the former and successfully in the latter. The chapter then turns to the first Vatican Council, where Cullen was an important voice in support of papal infalibility, ultimately providing the compromise language that was accepted as Catholic doctrine.