There are many ways to answer this because there were many reasons why some Americans disliked the Irish. It might be best for you to check your textbook and class materials to see what answer your instructor expects to see. For me, the best two answers would be A) not English and B) Catholic. Other possible answers are that the Irish were poor, uneducated, (believed to be) more likely to commit crimes.
People in the United States in the 1840s held many anti-Irish sentiments. As you can see from the cartoon in this link, Irish were often portrayed as subhuman and bestial. Their poverty and lack of education made them seem inferior to many Americans. As is the case with poor immigrants today, they were believed to be dirty, lazy, and prone to criminality. All of these are possible answers.
However, I think that there are two answers that are most likely to be correct. My second choice as an answer is that they were not English. The wave of Irish immigrants to the US was the first massive influx of immigrants. People were not as used to diversity in those days and they were suspicious of the Irish simply because they were different. One of the major differences was that they were not English.
My first choice as an answer is that they were Catholic. In those days, Protestants were very anti-Catholic. Among other things, they believed that Catholics could not possibly be good Americans. This was because they believed that Catholics would be loyal to the pope and not to their country. They also simply disapproved of the Catholic religion as there was a long history of anti-Catholicism in England going back at least to the time of Elizabeth I.
Thus, while there are many possible answers, my two top choices would be that they were not English and that they were Catholic.