I’ve been around Irish music all my life and I have never seen anyone play low D whistle in a group session. I’ve seen those things lying around at sessions, but not acually played. I’m thinking it’s like the Uilleann Pipes…something for professionals only.
Professionals? Not so. The reason may be more because the low D is hard to hear in larger sessions, and the player would not be able to monitor what he/she is doing. It’s not unusual to see them played in the sessions I frequent. There are abt. 3 individuals who bring them out now and again, but I said “see” them being played for the reasons I stated above. If the session is a small one, the low D can be better heard. I find that it’s at its best as a solo instrument or with minimal backup in acoustic situations where there’s no mixer to adjust things electronically.
Just my $.02,
N
I play low D in sessions - not all the time though, only when I feel like it. I believe that as a whistle player, one of my main consideration is not to be heard in sessions. If I wanted particularly to be heard, I’d play the fiddle or the pipes. I don’t find it difficult to monitor what I’m doing, and I’m not a professional (yet 
). In small sessions, I think it does add to the sound if used for some tunes.
Jo.
I’ve played my Copeland low D in sessions a few times, its the only one I’ve seen that is even remotely loud enough to hope to compete with other instruments. In a smaller session, its not too bad, but generally I’ll stick with a high D or flute.
Cheers,
Michael
I play my Low whistles all the time in session (A, G, F and D). I even play my bass-Bb now and again.
Need to get myself a low-C though…
I play my Kerry Pro Low D all the time in sessions.
Usually there are about 6 playing at any one time,
and it can usually be heard.
About 18 months ago, 30 people turned up with
instruments, and there was no way that a low D
would have been heard, because nearly everybody
played at once in a huge jam session,
so I used a high D … and got several complaints
that the pitch was ‘uncomfortable’ (peasants! 
)
So, I bought a Chieftain sop’ D - just to make sure
that they heard it properly 
Yes, I occasionally hear low Ds at sessions.
I play my low D in session on occasion. I don’t mind if it is heard seperately or not. I do get irritated by instrumentalists, particularly non-melody lines (the way I usually play the low D), who need to blare. Guitarists in my current session have a real bad time trying to be heard above all the other guitarists.
Occasionally the soprano D is heard over the rest of the group. I can only hope that I’m not screwing up when it is heard!
For the first time ever (that I can recall, anyway) I played my low D at the pub session last night. It was very quiet (for that pub) and the session was small and had more whistlers than fiddlers, so I played low for a couple of hours. Very fun. But hard, you need a lot more spots to breathe.
I play the low D quite a bit in sessions. Sometimes when there is another whistler or two, I prefer dropping to the low D, because B-parts of reels with two hi Ds that are almost in tune can be a bit grating after a while. Problem is that I find that I don’t practice the low D as much as the hi D and that I should if I want to play it more. I’ve never really had problems with the volume on either my Kerry (quite a tooter, that one) or the Overton.
Two thoughts on the tuning: If you have another whistler in the session and you are brave/foolish enough to try to tune, tune to the high A, rather than the low A. Most whistles are completely in tune between octaves and a bit of friction is easier to bear in the lower octave than in the upper.
Second thought: how do you get two whistle players to play in tune? Shoot one.
My ex-whistle & singing tutor Rory Campbell (from Belfast) regularly played an Overton low D in the sessions he founded in the Porterhouse in Covent Garden.
He never played the flute at all to my knowledge, though guitar was his main instrument. He played high D occasionally.
He played the low D in classes while everyone else played high (and didn’t that sound like a basket of cats having the op without anaesthetic) so we could pick out his tune rather than the infinite melodic & rythmic variations we used to come up with.