First found in county Cavan (Irish: an Chabháin) anciently the Eastern half of the Kingdom of Breifne, situated in the Northeast of the province of Ulster in the present day Republic of Ireland, where they held a family seat from very ancient times.

In the 19th century, thousands of Irish left their English-occupied homeland for North America. Like most new world settlers, the Irish initially settled on the eastern shores of the continent but began to move westward with the promise of owning land. The height of this Irish migration came during the Great Potato Famine of the late 1840s. With apparently nothing to lose, Irish people left on ships bound for North America and Australia. Unfortunately a great many of these passengers lost their lives - the only thing many had left - to disease, starvation, and accidents during the long and dangerous journey. Those who did safely arrive in "the land of opportunities" were often used for the hard labor of building railroads, coal mines, bridges, and canals. The Irish were critical to the quick development of the infrastructure of the United States and Canada. Passenger and immigration lists indicate that members of the Reilly family came to North America quite early: 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.

For more information on the last name Reilly, the PDF Surname History is available for purchase as well as other products.