The life of Riley
An easy and pleasant life.
The phrase originated in the Irish/American community of the USA, in the early part of the 20th century. The first printed citation of it that I have found is from the Connecticut newspaper The Hartford Courant, December 1911 - in a piece headed 'Bullet Ends Life of Famous Wild Cow':
The famous wild cow of Cromwell is no more. After "living the life of Riley" for over a year, successfully evading the pitchforks and the bullets of the farmers, whose fields she ravaged in all four seasons.
The quote marks that the writer added around the phrase are often an indication that the phrase in question isn't familiar to the readership, which is an indication of it being newly coined.
The phrase was much used in the military, especially amongst the Irish soldiery in WWI. The first known citation in that context is in a letter from a Private Walter J. Kennedy (who surely had parents or grandparents who hailed from the old country), 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":
"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) was 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.
There had been various Victorian music hall songs that had referred to a Reilly who had a comfortable and prosperous life; for example, there's the 1883 song, popularised by the Irish/American singer Pat Rooney - Is That Mr. Reilly? It included in the chorus "Is that Mr. Reilly, of whom they speak so highly?". Like most other Irish songs of the era, it played to the Irish audience - this one with a dash of anti-Chinese racism thrown in for luck (the Chinese were 'Reilly's' principal competitors for manual work in the USA at the time):
I'll have nothing but Irishman on the police
Patrick's Day will be the fourth of July;
I'll get me a thousand infernal machines,
To teach the Chinese how to die,
I'll defend working men's cause, Manufacture the laws,
New York would be swimming in wine,
A hundred a day will be very small pay,
When the White House and Capital are mine.
Another Irish/American sing, George Gaskin, was popular in New York around the same time. He was called 'The Silver-Voiced Irish Tenor", although audiences must have been rather forgiving in those days, as surviving recordings of him sound like a knife being drawn across a plate. 1897 song, The Best in the House is None Too Good for Reilly, elaborated on the whimsical idea of a wealthy Irishman being treated lavishly:
He's money for to pay,
So they let him have his way,
The best in the house is none too good for Reilly.
So, while the idea of a notional Irishman living the high life was current in late 19th century America, the phrase 'the life of Riley' isn't found until the early 20th century. It was clearly circulating in the language, by 1911, but it was probably the lyric of Howard Pease's popular song, My Name is Kelly, 1919, that brought it to the wider public:
Faith and my name is Kelly, Michael Kelly, but I'm living the life of Reiley just the same.
See other phrases that were coined in the USA.