The “Life of Riley” originated with the Irish-American soldiers in the US Army during WWI. The first known citation is in a letter from a Private Walter J. Kennedy, stationed at Camp Dix, New Jersey, which was published in The Syracuse Herald on 29th June 1918.

The piece was headed "Great Life, Writes Soldier at Camp", and the quote reads, “This is surely one great life." writes Kennedy. "We call it the life of Riley. We are having fine eats, are in a great detachment and the experience one gets is fine."

Later that year, on 22nd October, The Bridgeport Telegram published a letter from Private Samuel S. Polley, 102 Regiment, stationed in France, "They [German officers] must have led the life of Reilly as we caught them all asleep in beds..."

Who Riley (or Reilly, or Reiley) referred to isn't clear. If he had been a known individual then it surely would have been recorded. The lack of any such records points to the name being chosen as that of a generic Irishman, much as Paddy is used now.

The phrase may have been brought to America by Irish immigrants, although there's no known use of it in Ireland prior to 1918, or, more likely, it originated in the Irish community in the USA.