In most western societies, marital fertility began to decline in the nineteenth century. But in Ireland, fertility in marriage remained stubbornly high into the twentieth century. Explanations of Ireland's late entry to the fertility transition focus on the influence of the Roman Catholic Church in Irish society. These arguments are often backed up by claims that the Irish outside of Ireland behaved the same way. This paper investigates these claims by examining the marital fertility of Irish Americans in 1910 and produces three main findings. First, the Irish in America had smaller families than both the rural and urban Irish and their fertility patterns show clear evidence of fertility control. Second, despite the evidence of control, Irish-Americans continued to have large families, much larger, in fact, than the U.S. native-born population. The fertility differential between these populations was not due to differences in other population characteristics. Rather it was due to the fact that conditional on characteristics, Irish-Americans chose to have larger families. Third, the differential fertility patterns of Irish-Americans were not just due to the effects of being immigrants. Germans and English immigrants also had higher fertility than the native-born population, but to a much larger extent than for the Irish, this higher fertility could be explained by the population characteristics of these groups.