This is not a fluffy book. It requires actually thinking to read. It's worth the effort. The book is an analysis of the cultural stereotypes of "Paddy" and "Biddy" (or "Brigid") in the pre-Civil War USA. The point is to show that anti-Irish sentiment was not something driven by "extreme" elements, such as the Know-Nothing Party or nativist groups in general. Anti-Irish sentiment in pre-Civil War America was part of the self-definition of Americans. The book traces the development of this self-definition and how it contrasted itself against the "not-like-us", from the late Colonial period to just before the Civil War. It shows how the negative stereotype of Paddy started by using terms associated with upbringing and teaching. Paddy might come from Ireland, but his children could be made into good Americans (primary by becoming Protestant). By the 1830s, on the other hand, Paddy had become a fixed, racial stereotype. One inherited being Paddy as part of ones innate character, and this could never be "repaired". Knobel shows, in great detail, that this racist definition of "Irish as innate inferiors" was not a matter of agitation by extremists. It was part of the daily language of Americans, including Americans who would be considered "progressive" by present-day standards.
The book can be quite instructive on modern ethnic issues in the USA, although it could make many people, of both "right" and "left" political persuasions, quite uncomfortable.