Answer:
After the Great Famine or Potato Famine of 1845 to 1852, many Irish came to America looking for work and life opportunities. The situation was not unlike what America is facing right now with illegal aliens from South and Central America. They were taking most of the housing and jobs, which caused mass dislike of the ethnicity. Many businesses, refused to hire them posting signs saying "HELP WANTED: IRISH NEED NOT APPLY".