First found in county Cork, where they held a family seat from ancient times.

Suffering from poverty and racial discrimination, thousands of Irish Families left the island in the 19th century for North America aboard cramped passenger ships. The early migrants became settlers of small tracts of land, and those that came later were often employed in the new cities or transitional work camps. The largest influx of Irish settlers occurred with Great Potato Famine during the late 1840s. Although the immigrants from this period were often maligned when they arrived in the United States, they provided the cheap labor that was necessary for the development of that country as an industrial power. Early immigration and passenger lists have revealed many immigrants bearing the name O'Mahony: Cornelius, Daniel, Denis, James, Jeremiah, John, Mary, Michael, Patrick, Peggy, Thomas, Timothy and William Mahoney, who all arrived in Philadelphia, Pennsylvania between 1840 and 1860.

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