The "life of Riley" means to live lavishly without working. The origin of the phrase is under some dispute.
H. L. Mencken dates the phrase near the turn of the 19th century. He attributed it to a song called "The Best in the House is None Too Good for Reilly," written by the Tin Pan Alley team of Lawlor and Blake, who also created the immortal "Sidewalks of New York."
William and Mary Morris' Morris Dictionary of Word and Phrase Origins also says: "it comes from a song of the 1880s, 'Is that Mr Reilly?' popularized by Pat Rooney, founder of the great American song-and-dance dynasty The Dancing Rooneys." The Brewer's Dictionary of Phrase & Fable also supports this theory.
The historians of Fort Riley also claim, "The Cavalry School Hunt was officially organized in 1921 and provided a colorful spectacle on Sunday mornings. These activities gave rise to the perception of a special quality of life at Fort Riley that came to be known as the 'Life of Riley.'" While this phrase probably was used around this time, it's difficult to accept this as the origin of the phrase.