Where does the saying "Life of Riley" come from?

Reputedly from a popular song in the 1880s called, "Is that Mr. Reilly?".

The song apparently contained lyrics such as "a hundred a day would be my pay", and other speculation about what "Mr. Reilly" would do if he were to become wealthy.

However, the only source I have for this is one website, which is good enough for Government decisions on things like going to war but not usually definitive enough for AFT. Here's the link, though:
http://disappearingidioms.com/living-the-life-of-reillyon-easy-streetwhile-living-high-off-the-hog/#more-604 (another)

I have also found a reference to a song.
Riley (originally spelt "Reilly") referred to the character that appeared in a song written by Pat Rooney in 1890. In the song Riley says if he ever became the President of the US then "New York" would "swim in wine when the White House and Capitol are mine." It tells you something about the kind of President he would have made, doesn't it?

However it is possible it was an American poet by the name of James Whitcomb Riley who gave rise to this idiom. Apparently, Riley (1849-1916) wrote sentimental poems about young boys, which were very popular during his time. The poems told the stories of young boys lazing around during the summer without a care in the world — wandering about barefoot, swimming in the river and fishing.