Irish derivation: Ó Raghallaigh.

Name meaning: "Descendant of Reilly (extroverted one)."

Counties associated with the name: Cavan, Longford, Meath, Westmeath, Fermanagh, and Monaghan.

Coat of arms motto: "With fortitude and prudence."

Read more: What's your Irish clan?

Interesting fact:

 - Increasingly becoming a popular first name in the US.

- The O'Reillys were the most powerful sept of the old Gaelic kingdom of Breffny (Cavan and the surrounding counties), and the family is still prominent in the area.

Some famous O'Reillys:

- Bill O'Reilly (b.1949, American commentator and television show host of "The O'Reilly Factor" on the Fox News Channel

- John O'Reilly (Leader of The Saint Patrick's Battalion of Mexico)

- Sidney Reilly (1874–1925, Russian-British spy)

- Tony O'Reilly (b.1936, Irish billionaire businessman)

Read more: What's your Irish clan?