I knew there were ample primary sources out there on what the Irish thought about race and racial identity. And as I dug into them in the course of my own research, I realized how much these previous scholars had missed by not listening to the immigrants’ voices. I learned, for example, that the differences between whites (along Celt/Saxon lines) were just as important, in the minds of many Irish, as the differences between whites and people of color. Moreover, the Irish talked about identity in transnational terms; they thought of themselves as members of...