The Farrell surname comes from the Irish Gaelic name Ó Fearghail, which means "a valiant warrior." The surname Farrell was first found in Leinster, where they were found mainly in County Longford. Feargal, Prince of Annaly appears number 105 on the "Line of Ir" descendants. Ir was the fifth son of Milesius of Spain.

The most commonly-observed ancestry found in people with the surname Farrell is British & Irish, which comprises 58.4% of all ancestry found in people with the surname. The next two most common ancestries are French & German (16.6%) and Eastern European (5.8%).

The last name Farrell ranks 3,975th in popularity worldwide as of the 2014 Census and approximately 141,198 people carry the Farrell surname worldwide. The name ranks particularly high in the following six states: New York, California, Pennsylvania, Florida, Massachusetts, and Texas.

The name made it to the Caribbean as Farrill. The first of the line was Richard Farrill from Longford who appeared in Montserrat as an indentured servant in the 1670’s. These Farrills were later to be found in the Leeward Islands. America. Both Ferralls and Farrells exist in America, although Farrells outnumber Ferrells by approximately two to one.