Alcohol or, as Maguire calls it, “drink” was a problem for the Irish because they were not temperamentally equipped to handle it. Therefore, it affected them more than it affected any other “race” of people in the United States at the time.
According to Maguire, all Americans drank a lot in his day. The Irish, he says, did not drink any more than any other ethnic group. The problem was that the Irish were “mercurial and light-hearted” than the other groups who were “of hard head and lethargic temperament.” In other words, Irish people had a natural disposition that caused them to be more affected by alcohol than others.
This effect was, to Maguire, very bad for the Irish. It essentially ruined their chances at ever getting ahead in the US. He says that alcohol
deprives man of his reason, interferes with his industry, injures his health, damages his position, compromises his respectability, renders him unfit for the successful exercise of his trade, profession, or employment—which leads to quarrel, turbulence, violence, crime.
Since this happened to the Irish more than to others, it made them look very bad to other Americans. This is why alcohol was so bad for them.