Re: I thought it was the 19th-century song...
From
http://www.quinion.com/words/qa/qa-ril1.htm :
There are several explanations for this phrase, all of them centering on popular music.
William and Mary Morris point to a comic song written by the vaudevillian Pat Rooney in 1890 in which the hero of the song, a hotel-keeper named Reilly (or Riley), describes what he will do when he strikes it rich: New York “will swim in wine when the White House and Capitol are mine”. A version made famous by burlesque performers Ned Harrigan and Tony Hart has these lines in the chorus:
Well, if that's Mr. Riley
They speak of so highly.
Why, faith, Mr Riley,
You're looking quite well.
It was revived and updated in 1915 as a patriotic war song under the title Are you the O’Reilly? as an attempt to cash in on the success of It’s a Long, Long Way to Tipperary, and contained the chorus line “Gor blim me, O’Reilly, you are looking well”.
H L Mencken suggested as an alternative possible source The Best in the House is None Too Good for Reilly, which was written by Lawlor and Black at about the same period as Pat Rooney’s song.
On this side of the Atlantic, it is firmly believed that the song is of Anglo-Irish origins, and that the popularity of the phrase dates from a music-hall song My Name is Kelly written by Pease in 1919, which has the line “Faith and my name is Kelly, Michael Kelly, But I’m living the life of Reilly just the same”. Pease is here using a phrase which he obviously expected his audience to recognise, but we have no earlier recorded use.
Eric Partridge also thought the phrase is British and that it was taken up in America only in the 1930s, which would make the various US songs irrelevant as sources.
But we just don’t know the truth of the matter. The spelling of the name is as variable as that of the Irish surname itself, but Riley now seems to be preferred.
