There's an old Irish expression, something about "he left his fiddle
at the door." What is the full quotation, who said it and in what
context?

This might be helpful:

"There's an Irish saying about someone who is the life and soul of any
pub or party but unbearable at home: 'he leaves his fiddle on the
door'."

http://pubs.socialistreviewindex.org.uk/sr238/art.htm

You'll find some interesting material here:

"Hang up his Fiddle (To), to give a thing up as hopeless or as a bad
job; to decamp; to discontinue.

When a man loses his temper, and ain?t cool, he might as well hang up
his fiddle.?Sam Slick.

If a man at 42 is not in a fair way to get his share of the world?s
spoils, he might as well hang up his fiddle, and be content to dig his
way through life as best he may.?Dow: Sermons, p.78.

Hang up his Fiddle with his Hat (To) to lose all cheerfulness on
return home; to be merry abroad and morose at home.

Mr. N. can be very agreeable when I am absent, and anywhere but at
home. I always say, he hangs his fiddle up with his hat.?Theodore
Hook: Gilbert Guerney.

The Provencals have a proverb, Gau de carriers, doulou d?oustan
(?Joyabroad, grief at home?). (See Daudet?s novel Numa Roumestan. The
gist of the story turns on this proverb.)"

http://www.bibliomania.com/2/3/174/1118/14733/1/frameset.html