Famines are severe food shortages, and their history is as long as world history. Famines are caused by war, drought, earthquakes, and insects. One of the most important effects of famine is migration. People often move from one place to another because of famine.

Between 1845 and 1847, Ireland suffered a famine. Ireland was the most densely populated country in Europe. The famine was caused by a fungus that attacked potatoes, the main crop in the country. Potato rot caused the death of at least a million peasants and caused many of the Irish to leave their country.

By 1847, at least 100,000 Irish immigrants a year arrived in the United States. In 1851, the number of immigrants reached its highest at 221,000. The Irish included men and women who took jobs in cities, on construction gangs, as domestic servants, and in mills. Most of the new immigrants were so poor that they were destitute. Many were forced to settle in inner cities and live in tenements that were filthy and often dangerous.

By the 1850s, over half the population of Boston and New York City was Irish. The Irish often faced discrimination in their new country because of their Catholic religion, their socioeconomic status, and stereotypes about them. Over time, the Irish organized, became powerful, and gained better treatment through the vote.