First found in counties of Clare and Galway, where they held a family seat from very ancient times.

Many Irish Families did not fare so well within the English-ruled Ireland. Besides racial and religious discrimination, many families were renting out small tracts of farmland from absentee landowners at often unreasonable rates. Beginning in the late 18th century, moderately well off Irish families decided to emigrate to British North America or the United States in order to own their own plot of land. A radical change occurred in the 1840s, however, with the Great Potato Famine of the 1840s. Up to this point, the island's population had been increasing rapidly and a steady demand over the years for grain crops had depleted soil. Two failed crops and one poor one caused widespread disease and starvation. Thousands boarded ships looking for opportunities elsewhere. North America welcomed them as a source of cheap labor required for the many industrial and infrastructure projects underway, and as a means to quickly occupy the western regions. Research into immigration and passenger ship lists indicates that people bearing the name O'Halloran were among the earliest settlers to arrive in North America: Biddy, Ellen, Lawrence, Martin, Mathew and Michael Halloran all arrived in Canada in 1847; William O'Halloran arrived in Quebec in 1825; John, James, and Michael O'Halloran arrived in Philadelphia in 1876..

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