Popularized in and immediately after World War I in both Britain and America, due to troops mixing in wartime. Attested in 1918 in letters home by American servicemen, with post-war British usage popularized by song “My Name is Kelly” (1919), by Harry Pease, featuring the lines:

Faith and my name is Kelly, Michael Kelly,

But I’m living the life of Reilly just the same.

World War I spread possibly connected with the popularity of the song “Are You the O’Reilly?”,^[2][3] which featured a wealthy man living a life of comfort. The song was originally written in 1883 as “Is that Mr. Reilly?” by Pat Rooney Senior (1847–1892), a vaudeville performer, and featured an Irish character, then revived and adapted as a war song in 1915 by American P. Emmett with essentially the same lyrics, and as a British war song in the same year by Elwyn Yorke, with changed lyrics and an Australian character.^[4] The chorus runs:^[5]

Is that Mister Reilly, can anyone tell?

Is that Mister Reilly that owns the hotel?

Well, if that’s Mister Reilly they speak of so highly,

Upon me soul, Reilly, you’re doing quite well.

This proved very popular among troops, and may have influenced the spread.

Various theories of origin prior to this exist.^[1][2][6] The name is ultimately from the Ó Raghallaigh clan, and is one of the most common surnames in Ireland, ranking 8th in 1890, and Anglicized variously as O'Reilly, Reilly, and Riley. Due to its commonness, there is a high possibility of unrelated coincidences, and ultimate origin is hard to determine. In addition to the above song (from the 1883 version), another proposed origin is a different song “The Best in the House is None Too Good for Reilly” (1897), by Charles E. Lawlor and James W. Blake (origin proposed by H. L. Mencken), featuring the line:

He’s money for to pay,

So they let him have his way,

The best in the house is none too good for Reilly.

Other theories include the influence of the idyllic art of James Whitcomb Riley, or the historic wealth of the Ó Raghallaigh clan, due to their rule of East Breifne (present County Cavan) from the 12th century to early 17th century – see Kingdom of Breifne.