Isn’t it nice when tunes not originaly designed for “Irish” flute or whistle just fit on simple-system without too much hassle.
my two latest are:
I’d love to know of any other such melodies.
cheers.
edited for typos
Nabucco’s more fun than a barrel of puppies.
Can I add Procession of the Nobles to the fun list?
Pieces and parts of Bach’s Suite in B Minor are entertaining; his Partita is the half-holing exercise from Hades (my top speed so far on the whistle … probably a blazing 42! 
)
Mozart’s Concerti in D & G are good; Quantz has a few fun moments in his G Concerto, too.
Popp’s “Nightingale Serenade” might be in D, and as I recall from 11th grade, it’s a hoot on the picc, though I’ve never tried it on the whistle.
Finally, don’t forget Handel’s ‘Arrival of the Queen of Sheba.’ DeDanaan certainly didn’t, and it was brilliant.
The theme from Holst’s “Jupiter”.
How about Pictures at an Exhibition? The Old Castle (the Sax theme in Ravel’s version) is nice on low whistle and flute.
For those addicted to dots, collections of classical themes transcribed for oboe often come in the keys of D and G, and the range is similar, especially in the beginner oboe books.
Summertime, George Gershwin?
Actually oboes and 8-key flutes have a lot in common.
Forked F, long F, short F, G#, Eb, and of course the D
scale, other than C#.
Disclaimer: oboe since 1956 including 4 years in an
Air Force band, Boehm flute on and off since about
the same time, whistle since 1984, simple-system
flute since 2002. Assorted other instruments.
– Don
Can I add “What a wonderful world”, “Moonriver” and “What the world needs now”?