V. the commoner form Riley.

Reilly is a widespread Irish surname which comes from the Irish Gaelic O’Raghallaigh which means ‘descendant of Raghallach’ (‘the valiant or prosperous one’). Thus it is derived from an ancient Gaelic first name, and was probably given originally to a warrior (who would have become prosperous through booty).

The Reillys are an important family clan, which now has many subdivisions all over Ireland. However, the Reillys are still most numerous in County Cavan, where the main band of the clan is thought to have originated. The prefix ‘O’ is now being more frequently adopted in the anglicised form. This means ‘descendant of’ and is similar to the Scottish (and Irish) ‘Mac-’-‘son of’, and to Anglo-Norman ‘Fitz-’, and Welsh ‘Ap-’/‘Ab-’, though this last does not usually appear in the original form, as more frequently it has been absorbed. The most widespread example is in the Welsh surname Bevan, which derives from Ab-Evan.

The earliest record in English sources is in the Calendar of Patent Rolls, which shows a reference to one William Orailly in 1451, though in Ireland the name certainly goes back much further than this. The anglicised forms Riley and Ryley are borrowed from a native English surname based on a common place-name type (Devon, Lancashire, etc.) meaning ‘clearing where rye grows’.

Edward O’Reilly compiled one of the first Irish-English dictionaries in 1817.

Hugh Reilly (1630—95) accompanied James II into exile and wrote Ireland’s Case Stated Briefly, the only printed account of the state of Catholics in Ireland for a century.

Irish-born nineteenth-century newspaper editor and author Henry O’Reilly made a notable, if unusual, contribution to his adopted homeland when he contracted with inventor Samuel Morse to erect telegraph lines from Pennsylvania to the Great Lakes. He had strung over 8,000 miles of line before falling out with Morse, which event led him to abandon the project.

Hard-fighting US marine Lieutenant General William Riley, whose decorations included the Silver Star, the Purple Heart and the Croix de Guerre, proved to be an equally able diplo­mat when handed the tricky assignment of supervising United Nations military observers in potentially explosive post-war Palestine. The commission he headed tactfully settled over 1000 Arab-Israeli disputes and dealt with 65 cease-fire violations between 1948 and 1950.

Only one place name, Riley Hill, and no major geographic features in the United Kingdom relate to this name. It is equally rare elsewhere. The United States has 4 towns named Riley and a Rileyville and there’s a Reiley Peak in Arizona.

In England, Scotland and Wales Reilly is not common enough to be counted separately. In Ireland with about 27,000 name­sakes Reilly is the 11th most popular surname. Around the world Reillys are most common in Sydney (one in 875 families) and in Auckland (one in 1,113). In the United States there are an estimated 48,000 Reillys and 134,000 Rileys-in combination making this their 118th most popular surname.