First found in Munster, where they were seated from very ancient times.

Irish immigrants began to leave the English-controlled Ireland in sizable numbers during the late 18th century. Many of these Irish immigrated to British North America or the United States in the hopes of gaining their own tract of farmland. This pattern of migration grew steadily until the 1840s when the Great Potato Famine caused a great exodus of immigrants to North America. These immigrants differed from their predecessors in that they were desperately fleeing the disease and starvation that plagued their homeland, and many were entirely destitute when they arrived in North America. Although these penniless immigrants were not warmly welcomed when they arrived, they were critical to the rapid development of the United States and what would become known as Canada. Many went to populate the western frontiers and others provided the cheap labor the new manufacturing sector and the building of bridges, roads, railways, and canals required. A thorough examination of immigration and passenger lists has revealed some of the earliest people to arrive in North America with name O'Callahan or one of its variants: James Callaghan who settled in America in 1768; Gerhard Callaghan settled in 1764; James Callaghan settled in Pennsylvania in 1772; John Callaghan settled in Boston Massachusetts in 1764.

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