While the famine was responsible for a significant increase in emigration from Ireland, of anywhere from 45% to nearly 85% depending on the year and the county, it was not the sole cause.
The beginning of mass emigration from Ireland can be traced to the mid-18th century, when some 250,000 people left Ireland over a period of 50 years to settle in the New World.
Irish economist Cormac Ó Gráda estimates that between 1 million and 1.5 million people emigrated during the 30 years between 1815 (when Napoleon was defeated in Waterloo) and 1845 (when the Great Famine began).
However, during the worst of the famine, emigration reached somewhere around 250,000 in one year alone, with western Ireland seeing the most emigrants.

Families did not migrate en masse, but younger members of families did, so much so that emigration almost became a rite of passage, as evidenced by the data that show that, unlike similar emigrations throughout world history, women emigrated just as often, just as early, and in the same numbers as men.
The emigrants would send remittances (reaching a total of £1,404,000 by 1851) back to family in Ireland, which, in turn, allowed another member of their family to leave.

Emigration during the famine years of 1845–1850 was primarily to England, Scotland, South Wales, North America, and Australia; one city that experienced a particularly strong influx of Irish immigrants was Liverpool, with at least one quarter of the city's population being Irish-born by 1851.
Many of those fleeing to the Americas used the well-established McCorkell Line.

Of the more than 100,000 Irish that sailed to Canada in 1847, an estimated one out of five died from disease and malnutrition, including over 5,000 at Grosse Isle, Quebec, an island in the Saint Lawrence River used to quarantine ships near Quebec City.
Overcrowded, poorly maintained, and badly provisioned vessels known as coffin ships sailed from small, unregulated harbours in the West of Ireland in contravention of British safety requirements, and mortality rates were high.
The 1851 census reported that more than half the inhabitants of Toronto were Irish, and, in 1847 alone, 38,000 Irish flooded a city with fewer than 20,000 citizens.
Other Canadian cities such as Quebec City, Montreal, Ottawa, Kingston, Hamilton, and Saint John also received large numbers.
By 1871, 55% of Saint John residents were Irish natives or children of Irish-born parents.
Unlike the United States, Canada could not close its ports to Irish ships because it was part of the British Empire, so emigrants could obtain cheap passage (evicted tenants received free passage) in returning empty lumber holds.
However, fearing nationalist insurgencies, the British government placed harsh restrictions on Irish immigration to Canada after 1847, resulting in larger influxes to the United States.

In America, most Irish became city-dwellers; with little money, many had to settle in the cities that the ships they came on landed in.
By 1850, the Irish made up a quarter of the population in Boston, New York City, Philadelphia, and Baltimore.
In addition, Irish populations became prevalent in some American mining communities.

The famine marked the beginning of the depopulation of Ireland in the 19th century.
Population had increased by 13–14% in the first three decades of the 19th century; between 1831 and 1841, population grew by 5%.
Application of Thomas Malthus's idea of population expanding geometrically while resources increase arithmetically was popular during the famines of 1817 and 1822.
By the 1830s, they were seen as overly simplistic, and Ireland's problems were seen "less as an excess of population than as a lack of capital investment".
The population of Ireland was increasing no faster than that of England, which suffered no equivalent catastrophe.
By 1854, between 1.5 and 2 million Irish left their country due to evictions, starvation, and harsh living conditions.
While the famine was responsible for a significant increase in emigration from Ireland, of anywhere from 45% to nearly 85% depending on the year and the county, it was not the sole cause. The beginning of mass emigration from Ireland can be traced to the mid-18th century, when some 250,000 people left Ireland over a period of 50 years to settle in the New World. Irish economist Cormac Ó Gráda estimates that between 1 million and 1.5 million people emigrated during the 30 years between 1815 (when Napoleon was defeated in Waterloo) and 1845 (when the Great Famine began). However, during the worst of the famine, emigration reached somewhere around 250,000 in one year alone, with western Ireland seeing the most emigrants.
Families did not migrate en masse, but younger members of families did, so much so that emigration almost became a rite of passage, as evidenced by the data that show that, unlike similar emigrations throughout world history, women emigrated just as often, just as early, and in the same numbers as men. The emigrants would send remittances (reaching a total of £1,404,000 by 1851) back to family in Ireland, which, in turn, allowed another member of their family to leave.
Emigration during the famine years of 1845–1850 was primarily to England, Scotland, South Wales, North America, and Australia; one city that experienced a particularly strong influx of Irish immigrants was Liverpool, with at least one quarter of the city's population being Irish-born by 1851. Many of those fleeing to the Americas used the well-established McCorkell Line.
Of the more than 100,000 Irish that sailed to Canada in 1847, an estimated one out of five died from disease and malnutrition, including over 5,000 at Grosse Isle, Quebec, an island in the Saint Lawrence River used to quarantine ships near Quebec City. Overcrowded, poorly maintained, and badly provisioned vessels known as coffin ships sailed from small, unregulated harbours in the West of Ireland in contravention of British safety requirements, and mortality rates were high. The 1851 census reported that more than half the inhabitants of Toronto were Irish, and, in 1847 alone, 38,000 Irish flooded a city with fewer than 20,000 citizens. Other Canadian cities such as Quebec City, Montreal, Ottawa, Kingston, Hamilton, and Saint John also received large numbers. By 1871, 55% of Saint John residents were Irish natives or children of Irish-born parents. Unlike the United States, Canada could not close its ports to Irish ships because it was part of the British Empire, so emigrants could obtain cheap passage (evicted tenants received free passage) in returning empty lumber holds. However, fearing nationalist insurgencies, the British government placed harsh restrictions on Irish immigration to Canada after 1847, resulting in larger influxes to the United States.
In America, most Irish became city-dwellers; with little money, many had to settle in the cities that the ships they came on landed in. By 1850, the Irish made up a quarter of the population in Boston, New York City, Philadelphia, and Baltimore. In addition, Irish populations became prevalent in some American mining communities.
The famine marked the beginning of the depopulation of Ireland in the 19th century. Population had increased by 13–14% in the first three decades of the 19th century; between 1831 and 1841, population grew by 5%. Application of Thomas Malthus's idea of population expanding geometrically while resources increase arithmetically was popular during the famines of 1817 and 1822. By the 1830s, they were seen as overly simplistic, and Ireland's problems were seen "less as an excess of population than as a lack of capital investment". The population of Ireland was increasing no faster than that of England, which suffered no equivalent catastrophe. By 1854, between 1.5 and 2 million Irish left their country due to evictions, starvation, and harsh living conditions.
Great Famine (Ireland)