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

A great mass of Ireland's native population left the island in the 19th century, seeking relief from various forms of social, religious, and economic discrimination. This Irish exodus was primarily to North America. If the migrants survived the long ocean journey, many unfortunately would find more discrimination in the colonies of British North America and the fledgling United States of America. These newly arrived Irish were, however, wanted as a cheap source of labor for the many large agricultural and industrial projects that were essential to the development of what would become two of the wealthiest nations in the western world. Early immigration and passenger lists indicate many people bearing the O'Rielly name: the families who settled in Newfoundland - in St. John's, Tilton Harbour, Placientia, Tickle Bay, Tilt Cove, and many more, between 1805 and 1871; James O'Reiley settled in New York in 1823.

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