What’s the origin of "the life of Riley"?

Dear Straight Dope: So, who’s Riley and why is it so prestigious to be living his life? fromhell@swbell.net

Dex replies:

William and Mary Morris’ Morris Dictionary of Word and Phrase Origins says:

There are several theories of the origin of this popular catch phrase which means, of course, to live luxuriously without working. One authority 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.”

Brewer’s Dictionary of Phrase & Fable supports this notion. The Morrises continue:

However, no less an authority than H. L. Mencken is quoted as dating it near the turn of the [nineteenth] century and attributing 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.” Be that as it may, the phrase came into wide favor in the early 1900s and later–with Reilly simplified to Riley–became the title of a popular television program.

Dex

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What’s the origin of "the life of Riley"?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD