Oh, I certainly didn’t mean to sound pejorative, because I like the overall effect, and yeah, the sound can be filled out with mandolin / guitar etc, but I was just curious and wonder what the reaction would be to someone on a whistle trying a harmony in a session 
Hoopy
That would depend upon the session and how good the whistle player was. Unless you’re firmly rooted in the tradition and are one heck of a player, it’s best not to go messing around in a session setting. Odds are, someone would think you’ve jumped off onto another tune and get confused…I know I would.
Eric
And the Union Jack T-shirt’s a no?
Stay hoopy!
We have a whistle player who plays whistle exclusively If the whistle player is there I don’t play my whistle. He is good and that’s his instrument and if I play along it’ll only mess things up. We have another flute player and it seems ok if both of us play the flute at the same time. Flutes are quieter.
There is one lady who is a beginner on whistle and fiddle. When she plays the whistle it is awful. So the idea that it’s a beginner instrument is definitely an error in judgment.
Flutes are quieter
I’ve seen only two sessions - Moscow session which I attend regularly and Kiev session, where I’ve been only once.
Moscow session was started a year ago as a session of tinwhistle.ru community, and at first there were ONLY whistles there. Four, five, six whistlers. Sometimes low whistlers. And sometimes no bodhran or guitar. Then things a bit changed, and in the beginning of the year I’ve found myself the only whistler among 4 fiddles, 4 flutes and low whistles, one bodhran, two guitars and two bouzoukis.
This summer the session was pretty dead, so I don’t know who will come up in autumn.
Kiev session had three or four whistlers, one of them (Rut) was great, others were either good or mediocre.
I haven’t played much with a simple-system flute, but I know that there are definately differences in Boehm flutes. On my Gemeinhardt you really have to push to hit a middle C# in tune; on my friend’s Pearl it feels like you have to hold back. Other notes have similar issues. Maybe the differences aren’t as strong as on simple-system instruments, though.
Often not welcoming. I’ve done this, but only sparingly, and I stop if I get a
dirty look. Often, because of the open harmonic structure, a guitarist (or
other accompaniest) has several choices of chords, which can alter the
harmonic structure of a tune. You’d have to make darn sure the harmonies
you’re playing agree with what the guitarist is doing.
‘Sparcely’ I love that word!! A cross between sparse and scarcely. 
Well if there’s no guitarist, then not much else to worry about.
Sure, I understand how harmonizing might clash with that chord substitution that the guitar/bouzouki/keyboard player decided to throw in on the spur of the moment.
But, still, when a good fiddler happens to play some double-stops, that sounds particularly sweet, to my ear, and surely a double-stop must count as harmonizing?
I just realized that now! I thought it sounded funny to begin with…