Anyone have any tips or internet resources for using whistles for playing harmony rather than melody?
You really don’t hear this too much in Trad music which is too bad because harmony is really wonderful. Check out Phil Hardy’s Whistleworks album for some really nice harmony playing. As far as tips go, I think you’re pretty much on your own. Find a recording of a tune you like, grab the appropriate whistle, and go to it!
just like harmony on any instrument - if you know the chord that is at that particular place in the music, you have at least three notes to choose from that will “work”.
For instance, if the chord at that place is a “D”, you can play a D, F#, or A note and it will “fit” (harmonize). These are the three notes in the triad of the chord. If the chord is a D7, you could also have a C#.
Just noodle around with the triad notes of chords and you’ll come up with some great harmonies. Also work on notes that transition between chords (these often sound good in a “minor” flavor).
By the way - this type of playing also works if you only have one key of whistle and the song that is being played is in a different key.
Missy
Missy’s advice is good, another harmonic technique is to play the tune patterns a 3rd up or down in the same scale. The 3rd down can give you an interesting relative minor version of the tune, which you can harmonize with appropriate chords.
John S
That would be a C natural. I imagine that was just a typo, but since the C# would change the flavor so much–significantly away from the Irish Trad sound–I just thought I’d point it out. The C# would give you a Dmaj7, a chord like the first one in “Misty.”
My own preference for harmony, though far from traditional, is countermelodies with Irish traditional tunes, especially hornpipes and some jigs, and especially with things that give a bit of a fugal feeling. Here’s a small sample. You’ll hear a few measures of unison whistles then the counterpoint in the repeat (Pipe on the Hob).
These days, though, I prefer the tune straight.
Carol
whoops sorry - got my half and whole steps mixed up!
(I very rarely worry about 7ths since I’ve only got three strings on the dulcimer!)
Actually, even if the chord isn’t a D7 in many cases it works just fine to make it a D7 by playing the Cnat or even a D9 by playing an E, etc. After all, most instruments aren’t capable of playing chords and in orchestras the various instruments playing different parts are used to make the “chords” of the song.
You also don’t have to play all the notes of a chord to “suggest” that chord. For example, playing D, A, C will be heard as a D7 even though the third (F#) is missing. (Missy, keep this in mind on your three string dulcimer 
.)
Obviously, you will change the “flavor” of the song by making such substitutions but it’s not really unfair to say that you are getting away from ITM anyway as soon as you start harmonizing a tune - to what degree you want take that variation is up to you.
I just got “The Complete Idiot’s Guide to Music Theory.”
I am really enjoying this read. Learning lots of things I have always wondered about for many years. It is still quite technical and so I am wondering if I really need
“The TOTALLY Complete Idiot’s Guide to Music Theory” 
Whistle = Trad?
could use a copy of that myself, actually.. 
Assuming you know a little bit about theory and counterpoint, harmony playing on whistles is just like harmony playing on other melody instruments. Saxophone, trumpet and trombone sections in big bands give some idea of what you can do but really anything that sounds good is OK. What you might use as amodel depends very much on teh style you ar playing.
I do a lot of harmony playing and recording in non-Irish contexts—various styles of rock, blues, jazz, folk music. Choosing the whistles to use is a lot of fun. I’ll sometimes choose to get a perfect match in sound—I might use three Overtons playing different parts, eg, high D, A low D. Another combination I like is a fairly flutey whistle, say a Copeland low, with a relatively pure whistle, a Burke, a Sindt or a Busman. If you have a recording setup that allows you to layer parts you can have a lot of fun.
Not necessarily but not excluding Trad. Of course, I’m aware that to many as soon as you include harmony it ceases to be IrTrad. I prefer to keep all my options open. There is no way I could limit myself to just Trad!!
Now that’s exactly where I was coming from. Thanks Wombat.