Irish emigration across the Atlantic began long before 1800. In the 1600s, approximately 25,000 Irish Catholics left – some were forced to move, others left voluntarily – for the Caribbean and Virginia, while from the 1680s onwards Irish Quakers and Protestant Dissenters began to depart for the New World. Considerable Presbyterian emigration from Ireland's northern Province of Ulster took place from the 1710s onward, alongside smaller Anglican Protestant and Catholic emigration from Ulster and the southern province of Munster. This pattern continued until the end of the Napoleonic Wars in 1814. Ireland had benefited considerably from price rises associated with war on the European Continent, only to suffer as a result of the drop in export price levels following the Battle of Waterloo. From 1815 to the start of the Great Irish Famine (1846–1852), between 800,000 and one million Irish sailed for North America with roughly half settling in Canada and the other half settling in the United States. Significantly, no other European country contributed as many emigrants per capita to the New World as Ireland during this period. Until the early 1830s, Protestant departures exceeded the number of Catholics leaving Ireland. Thereafter, Catholics greatly outnumbered Protestants. The demise of the cottage spinning industry in the first half of the 19th century – especially from the early 1830s onwards – led to a massive displacement of workers. Nonetheless, the rise of the linen industry in east Ulster, which was able to compete successfully with Lancashire factories, meant that migration in Ulster was predominantly from the countryside to Belfast. No comparable industrialization took place in southern Irish towns and cities, which meant that most people in search of work in rural Ireland had to emigrate across the Atlantic or, to a lesser extent, across the Irish Sea to find employment.
Most of the approximately 1.8 million Irish who arrived in the United States between 1845 and 1855 – before, during and immediately after the Great Irish Famine – came from much poorer backgrounds than the Irish migrants who had gone before them; for example, almost one third of the new arrivals during this period originated from the poorer Gaelic-speaking regions of Ireland. The extreme conditions of the Famine period understandably provided strong motivation for many Irish people to leave Ireland. Yet, the outward flow continued after the Famine and throughout the second half of the 19th century because of the decline in domestic industry, the shift in farming from tillage to pasture, and the increasingly impartible nature of Irish inheritances whereby farms were passed on in their entirety to the eldest son rather than being divided among all sons. Between 1850 and 1913, over 4.5 million Irish people left their homeland.

High mortality during the Famine and the continuing trend of large-scale emigration set in train by the Famine saw the island's population drop dramatically from a high of almost 8.5 million in the mid-1840s to 6.5 million in 1851, and to 4.4 million by 1911 Post-Famine emigrants departed for more varied reasons, including the declining demand for agricultural labour, the fall in wages in Ireland relative to the United States, the desire to avoid or postpone marriage, and the accumulation of extensive contacts between Ireland and the Irish community in North America
People of all religions continued to depart the newly independent southern Irish state even after it gained its independence from the United Kingdom in 1922. Catholics left because of the prospect of greater prosperity elsewhere; Protestants emigrated because of the prospect of an Irish Free State dominated by the Catholic Church and more limited economic opportunities. A major shift in the direction of Irish emigration occurred from the mid-1920s onwards due to new American immigration quotas and the effects of the Great Depression in America throughout the 1930s. Consequently, many Irish people chose instead to move to the United Kingdom, as no travel restrictions applied between the two countries.
What makes the period between 1850 and 1914 so important, however, is that, in contrast to previous European migration, it represented a period of extraordinary migration across the Atlantic. Indeed, over 50 million Europeans migrated to North and South America between 1850 and 1914. Ireland, Italy and Sweden contributed over 15 million of these emigrants; and more departed beforehand, especially from Ireland. In total, over 6 million Irish people headed across the Atlantic in the hundred year period between the end of the Napoleonic Wars and the start of the First World War. Considering Ireland's population reached a peak of approximately 8.5 million before the Great Famine and fell to 4.4 million by 1911, the number of Irish people leaving for North America relative to the population of the island throughout this period was extraordinary. The rate of emigration from Ireland was more than double that of any other European country. Italy also provided millions of migrants to the New World during this period. Indeed, Italians made up the largest group of migrants who came to America from 1880 to 1920. Of the approximately 20 million Italians who left their country between 1876 and 1950, around 10 million migrated to the Americas (about 5.5 million to North America and 4.5 million to South America). While proportionally less Italians than Irish emigrated, the sheer numbers of Italians crossing the Atlantic meant that they too had an enormous effect on North American culture; and also – unlike their Irish and Swedish counterparts – on South American culture, particularly in Argentina and Brazil. Although not as large in terms of overall numbers, one in five Swedes migrated to North America between 1840 and 1930, amounting to 1.2 million people overall.
People left Ireland, Italy and Sweden for similar reasons, although the Great Irish Famine represents a notable exception. Most migrants moved to improve their economic status. Religious persecution played a minor role in early Swedish emigration, but numbers leaving for this reason remained small. Similarly, some Irish and Italian migrants left for political reasons, but this type of migration again represented a fraction of overall figures. Most people departed to escape poverty because of the limited economic opportunities available to them in their home countries. In Ireland, land reforms after the Famine often meant that only one son received the family farm, effectively making the other male siblings landless labourers. This also made it increasingly difficult for young Irish women to marry a man with land. Given the large size of Irish families, these people made up a sizeable portion of the Irish population. The availability of higher wages in America compared to Ireland, as reported by family members and friends through letters and remittances sent from abroad tempted many young Irish people to depart their native land.
The majority of migrants from Ireland, Italy and Sweden who travelled across the Atlantic were young, single and unskilled. Irish Presbyterian and Swedish families who set up farms in the American Midwest in the early-19th century and in the second half of the 19th century respectively were notable exceptions to this trend. Young, single people with little work experience had less to lose than their older, more skilled counterparts who had to provide for their families. During the first half of the 19th century, males dominated Irish emigration by two to one. After 1850, however, rural Irish women experienced a significant erosion of their socio-economic status, prompting considerable migration. The choice for young single Irish women often lay between becoming an independent worker across the Atlantic or an unpaid helper in her family home in Ireland, and many chose to emigrate as a result. Indeed, after the Great Famine, Irish males and females travelled in roughly equal numbers across the Atlantic – a pattern that was closely mirrored by Swedish emigration. Chicago had a large surplus of Swedish women, who often worked as maids, whereas in the American countryside there remained a shortage of Swedish women compared to the number of male Swedish farmers. Approximately half of Irish female emigrants in America worked as domestic servants in cities. Mixed migration led to the formation of lasting Irish and Swedish communities. By contrast, males dominated Italian emigration across the Atlantic by roughly four to one. Italian Catholic culture allowed women less freedom than Irish Catholic culture, so that Italian women were discouraged from emigrating to become domestic servants similar to Irish and Swedish women. Another reason for the preponderance of Italian males related to the high rate of return migration. Some Italian men already had wives, who remained in Italy awaiting their husbands' return. After 1900, Italian women did begin to travel in larger numbers; nonetheless, they remained in the minority until after 1913, when they began to outnumber men.
The extent of emigration across the Atlantic from Ireland, Italy, Sweden, and Europe generally between 1800 and 1950, and especially from the 1840s until the outbreak of the First World War, was truly momentous. The great exoduses from the three countries represented different phases of the European Atlantic migration: Irish emigration peaked in the middle of the 19th century, Swedish emigration reached its height in the 1880s, and Italian emigration climaxed in the early 1900s. Never before had so many mainly young, unskilled and single Europeans travelled such long distances to start new lives. Most left Ireland, Italy and Sweden because of the poor rural conditions and limited opportunities for improvement at home compared to the apparently plentiful possibilities and rich rewards promised abroad. Although many emigrants initially encountered hardship on the other side of the Atlantic, they saw the potential for improvement, which stood in stark contrast to the stagnant world they remembered back home. For Swedish and Irish migrants, the other side of the Atlantic nearly always meant the United States and, to a lesser extent, Canada. For Italians, however, it meant the United States and various South American countries, especially Argentina and Brazil. Italy's emigrants were predominantly male, whereas the Irish and Swedes recorded a more balanced rate of departure between the genders. One reason for this related to the Italians' high rate of return compared to their Irish and Swedish counterparts, which also led to a slight delay in establishing fixed Italian communities abroad