Because the whistle comes in the many various keys, do players seem to use half holing or shading to produce whatever flats or sharps that are not included in the specific whistles key? Thanks.
Bryan
Well, that sort of depends. If you’re trying to play Cn, for instance, (the most common out-of-the-key note because it’s used to play in G) then you could half-hole it, as some players and professionals do, or you could use the (probably) more-common way of cross-fingering.
The cross-fingering for Cn on a D whistle is oxxooo or oxxxox or something similar. Depends on what whistle you’re using as to which is in tune. Other notes can be cross-fingered, such as Bb and sometimes G#, but something like Eb or Fn couldn’t be cross-fingered because there aren’t enough unused holes at the bottom of the instrument to shade them.
But, the good news is that Irish music only has the occassional accidental, so half-holing is typically a non-issue in that regard. If you’re playing other types of music, though, this could obviously become an important thing.
Half holing is a very useful thing to learn. On a D, learn to half hole a Cnat, Fnat and G# and you’ll find them useful from time to time. Takes a bit of practice, and it differs from whistle to whistle.
It’s good to learn to half-hole Bb too. It’s indispensible for blues in G.
Aldon
Or E, for that matter.
I also find half-holing to be useful… and more natural than adding another fingering into the mix (just the way I learned it).
For C natural on a D whistle - hold as for B, then just roll the whistle a little to the right to open the hole slightly without refingering. Practice this to get a reliable clear tone.
(this assumes the standard left hand on top, right hand on bottom hold, either way, roll to the right for notes on your left hand, the opposite for notes on your right hand).
Edited (with thanks to congrats!))
Fixed! 
Thanks for that! (sound of palm hitting my forehead)
St Anne’s reel is a good one to learn to half hole the G#.
Any tune in C (no sharps) is great for F half-holes. On Slow, expressive airs, that portamento is really great. Just remember: when you’re sliding up to a note, you’ve got to get all the way to that note! Some very well known players (Da$%y Spi#$$%^e included) slide to something that’s bloody out of tune.
Spleen vented. Feeling much better now.