I’ve been resisting WOAD for a long time, but I just can’t stand not playing the tunes that are in a bad key or range for my beloved soprano D whistle. A soprano E might be good for tunes in A. Soprano C whistles are common, but I haven’t run across that many tunes in C or F. An alto A or G might be good for the tunes that dip below D. I am hoping that those more experienced than me would be able to rank the usefulness of non-D whistles for session play in the States.
Thanks,
ObieWhistler
C whistle is useful when playing with fiddlers who like to play in Dm, F, and Gmaj/min/mix (same as Em, G, and the various A fingerings on a D whistle, respectively). Fiddlers love them some Dm. 
An E whistle would be good particularly when among Scottish players, as it seems. There are a few E major tunes out there in their selections, such as The Wedding Reel and Calliope House, whereas I’ve known Irish-style players to usually play those in D. It would at least be good diplomacy, anyway. 
If you learn to play G#s on your D, that opens you up to the key of A without having to buy a new whistle or switch mid-set.
A very few whistles give good G#s in both octaves with xxo xxx (low) and xxo xox (high). If yours doesn’t, xxo xxo might work for high G#. Half-holing, as in xx/ ooo, works for both octaves on any whistle.
In the few sessions I’ve been to, the tunes that strayed away from 1 and 2 sharps strayed towards the flat side: D dorian, D minor, G mixolydian, G dorian. They would have worked well on a C.
Session players usually stick with a D whistle because that’s the one you need to play all that Irish stuff 
. At our session there are more songs that require other keys so bring a bag of whistles or just sit there with your beer. A C whistle is a good place to start IMHO. Recently I found that a low G whistle can be used to play in Em (in the Bm position of a D whistle) to play in a lower register but with limited high notes. Some people play the same fingering in lower keys for soling just because it sounds different than a soprano D whistle.
Yes, you guessed English is my second language or it is just late!
Cheers
Nate
Actually, the G#s in tunes in A are rarely crucial to the tune and can most often easily be avoided. If there are any at all.
FWIW, I find a C whistle more important because tunes in Dm/Ddor also like to drop to the low C, which I’d consider more essential.
I’d recommend to all beginners who are looking into trying more keys to get a full set of generations. That will cover your bases from Bb all the way up to Eb, and is not so expensive.
a full set of generations. That will cover your bases from Bb all the way up to Eb, and is not so expensive.
Discard the F and G then?
This is going to be highly dependent on your session and what tunes you play. Trying to put rough numbers on it, in sessions I use a D whistle 95% of the time, E and A whistles 2% each, and a C whistle maybe 1%. If I’m not hauling my full set of whistles to a session, I usually bring two D whistles (high and low) and don’t worry about other keys.
Alas, I learned to play clarinet, oboe, and saxophone nearly fifty years ago, and I struggle with exotic fingerings such as half hole and G# on a D whistle, which would simply be absurd on those instruments. I manage the C naturals alright, but the rest feel unnatural. I guess my previous training has left a permanent mark on me, and I imagine that alternate fingerings might be easier for someone who starts on whistle to begin with. Hence my desire for whistles in other keys.
“I struggle with exotic fingerings such as half hole and G# on a D whistle”
FWIW- in our little traditional Irish band ,apart from a D I use the following most often (in this order): A,G,Low D, Bb,C
This takes care of our repertoire of Irish and Scottish dance tunes, pub songs etc.
Well, at very least the G , yes, might be best
But no, in all seriousness, I was pretty tired when I wrote that. I meant to include all of the keys and just typed it wrong.