Small numbers of Scots Presbyterians from Ulster had emigrated to America as early as the late 1600s, mainly from the Laggan area in northeastern Donegal.  In 1718, their exodus began to increase significantly, when 11 ministers and nearly 300 members left Derry in five ships bound for Boston.  The Aghadowey congregation under James McGregor settled Londonderry, New Hampshire.  Another ship left Derry soon after, and settled in Casco Bay, Maine.  At this time, Scots immigration into Ulster had almost ceased.  3,500 Presbyterians left Ulster for America between 1725-1727, and reached a peak in 1728-29.

The Scots who settled in the province of Ulster in Ireland were perhaps the first group to begin a wave of migration from Ireland to America.  Having been encouraged to migrate to Ireland by grants and low rents, they experienced poor harvests, religious discrimination due to their refusal to convert to the Church of Ireland, and high rents.  They left Ireland for America at a rate of about 4,000 per year.

In the 1770s, emigration to America again increased to about 10,000 per year as rents rose, harvests failed, and the linen trade slumped. Emigration from Ireland almost ceased with the beginning of the American Revolution.

The unsuccessful rebellion of 1798 affected people throughout Ireland.  Although dissatisfaction with England's rule was expressed mainly in the south and west of the country, there was measurable support for the failed rebellion in the Ulster counties of Antrim and Down.  They people were primarily Protestants, but there some were Catholics as well.

In 1798, over a span of about three weeks, about 30,000 people throughout Ireland were killed in the rebellion.  Many were armed only with pitchforks and pikes, and some were women and children.  Following the end of this uprising, the commander of government forces in Ulster issued a general amnesty to the rebels in Antrim, however those of Down were shot.  The two main Ulster rebel leaders were executed.  These actions promoted a widespread fear of further reprisals and repression, and encouraged migration.

The 1801 Act of Union between Great Britain and Ireland forced wealthy Protestants to give up their power, while Catholic emancipation was offered to the majority, although it never really was realized.  The loss of Ireland's parliament further weakened the home industries, while augmenting competition from English factories.  Thus, the union of Ireland with England generated further discontent among poor Protestants, and heightened sensitivities to the continued oppression of Catholics.

Emigration to America reached a peak as a result of the devastating potato blight that first struck in 1845, and returned in 1846, 1847, and 1848.  Although the majority, conditioned to scarcity and want, managed survive the first outbreak, a terrible famine ensued as the blight returned to affect nearly the entire island.  More than one million people died from starvation and disease.  The British government did little to help.  Many who survived were helped by individuals and private charities.  More than one million people emigrated during those years.

Those who chose to migrate to America, rather than Canada or Australia, were attracted to the major port cities of Boston, New York, Philadelphia, Charleston, and New Orleans.  Each of these cities had its own advantages and disadvantages.  Philadelphia, particularly, presented opportunities, because it was a thriving industrial city, with jobs available to both the skilled craftsman and the general laborer.  Equally attractive was the availability of housing for purchase at reasonable prices or for cheap rents.  The Irish stressed the importance of owning their own homes, and many achieved that after not too many years.  Philadelphia also offered one additional attraction to the Irish immigrant - it was the cradle of American freedom from British colonialism.