When a potato disease (blight) arrived in 1845, possibly in ships from America, it was a disaster. Potatoes went rotten, and were not fit to eat. People went hungry. More than 1 million people starved to death. Many more got sick. In Ireland, one in four people (25%) died or emigrated. The potato famine was one of the most terrible events in Irish history.

Read more here: http://www.bbc.co.uk/schools/primaryhistory/victorian_britain/famine_and_emigration/

The voyage across the Atlantic Ocean could be terrible. The ship might be at sea for a month. Food was bad and the overcrowded conditions meant lots of people got sick and died.

Migrant ships were called 'Coffin Ships', because so many people died. It's thought that in 1846/1847 20,000 people died in migrant ships sailing to Canada alone!

Source:
http://www.bbc.co.uk/schools/primaryhistory/victorian_britain/famine_and_emigration/
Image source: The Illustrated London News, May 10, 1851

The entire caption reads “Here and There; or, Emigration a Remedy.”

Source: https://viewsofthefamine.wordpress.com/punch/here-and-there-or-emigration-a-remedy/

First of all, laissez-faire, the reigning economic orthodoxy of the day, held that there should be as little government interference with the economy as possible. Under this doctrine, [it seemed unacceptable] to stop exporting Irish grain to Britain.

Recent historians of the famine [...] have been inclined to stress the important parts played by two other ideologies of the time: those of 'providentialism' and 'moralism'. […]

There was a very widespread belief among members of the British upper and middle classes that the famine was a divine judgment -an act of Providence- against [the traditional Irish way of farming] that was believed to have given rise to the famine. [...]

Finally, educated Britons of this era saw serious defects in the Irish “national character” -disorder or violence, filth, laziness, and worst of all, a lack of self-reliance. This amounted to a kind of racial or cultural stereotyping. The Irish had to be taught to stand on their own feet and to unlearn their dependence on government. [Bringing help from Britain seemed therefore counter-productive].”

The immigrants took whatever unskilled jobs they could find, working on the docks, pushing carts, or digging canals and laboring on the railroads. Their lives were so harsh that their mortality rates remained high. For example, 60 percent of children born to Irish immigrants in Boston died before the age of six. Adult immigrants lived an average of only six years after their arrival in the United States. When these immigrants arrived, they were a comparatively docile and law-abiding population. However, many of them turned to crime out of boredom, desperation, and anger. Young Irish immigrants in New York City formed criminal gangs, and the area known as Five Points became a cauldron of all manner of criminal activity.

Source: http://immigrationtounitedstates.org/528-great-irish-famine.html