The name is extremely common and widespread throughout Ireland, ranked 8th most common in 1890 and 11th in 1996. Its origin is in the old kingdom of Breffny, which included areas now in counties Cavan and Longford, where the O'Reillys were long the dominant family, despite many attempts by their main rivals, the O?Rourkes, to make it otherwise. The inauguration place of the family was the hill of Shantemon in Castleterra parish. They were renowned in medieval Ireland for their involvement in trade; their success may be gauged by the fact the "reilly" was at one point a colloquial term for money in Ireland. What use they made of their prosperity can only be conjectured, but the phrase "the life of Reilly" is suggestive.
After the collapse of Gaelic power in the seventeenth century, large numbers emigrated to serve in the armies of France, many in Colonel Edmund O'Reilly's regiment of foot. The connection with the original homeland is still strong, however; even today (O')Reilly is the single most numerous surname in both Cavan and Longford.
The return of the prefix has been spectacular. Less than 10% give their name as "O?Reilly" in 1890, but almost 60% in 1996.
Count Alexander O?Reilly (1722-1794) was born in Co. Meath and fought in the armies of Spain, becoming Governor of Madrid and Cadiz, and Captain-general of Andalucia. His later career took him to Cuba, where many O?Reillys are still to be found. His name is recorded in Calle Orely in Havana.
John Boyle O?Reilly (1844-1890) was also born in Co. Meath. After transportation to Australia for his membership of the Fenians, he escaped and made his way to the U.S. in 1869, where he became proprietor of the Boston Pilot.
A.J.F. O?Reilly (1936 - ) is the best-known contemporary bearer of the name. Continuing the trading tradition of the family, he is President and Chief Executive of the Heinz Corporation, as well as having extensive personal business interests worldwide, including effective control of most Irish national newspapers. He played international rugby for Ireland for 16 years from 1955 to 1970, winning 29 caps.