Pipers often play low whistle (Paddy Keenan, Davy Spillane, John McSherry, Brendan Ring etc) , but I wonder why some famous wooden flute players like Mike McGoldrick, Brian Finnegan, Seamus Egan, Alan Doherty etc. or some C&F members here play Low Whistle as well?
Because they can. 
If Darwin hadn’t intended us to play low whistle, he wouldn’t have invented Bernard Overton.
To attempt a serious answer; there’s a difference in tone, it gives a variety to the ergonomics of playing, and low whistles are just too wonderful not to play even though it could be argued that their niche in the soundscape is already occupied by flute. Cormac Breatnach switched completely to low whistle from flute and there must be others.
I very rarely play concert flute but extremely regularly give my low whistle a run out; apart from the fact that I love my Goldie/Overton Low D, it’s also personally because I play other instruments and play a lot of fife, and (let’s face it) just don’t put enough time into maintaining my flute embouchure when I’ve got the low whistle ready and raring to go. I’ll occasionally work on my flute embouchure and then the fife lip goes to pot, and of the two it’s the fife embouchure I need most and therefore save first.
Hope that gives some sort of answer 
I don’t - never felt the need if I have the flute.
I switch to low-D for a couple of reasons: first, as mentioned above, the tone is distinctly different. Second, my group (Gallowglass in Wheeling, WV - we actually trademarked the name!) seems to play quite a few 2+ hour gigs and my chops can get a little knackered after a while and the whistle takes a lot less physical effort than the flute. The second, to be honest, is my main reason - it gives me a little break without having to sit there and do nothing…
Pat
Ditto. All the years I was a fluteplayer I had no interest in Low D whistles.
I had flutes in Eb, D, and C.
I only played whistles when our band did songs requiring keys/ranges I couldn’t cover with my flutes. So I had a Low E whistle, and mezzo whistles in G and A.
This lack of interest in Low D whistles changed when I stopped playing flute altogether, and took up the Low D Whistle more or less as a flute subsitute.
[Thread revival. - Mod]
I was an early adopter, and may have started it seriously in a session with Paddy Moloney in April 1986 at the Edinburgh Harp Festival. The Chieftains were opening the Folk Festival which follows, and he’d wandered through the bar, when his eyes fell on what was sticking out of my pack. Two of the early Dixons, one pure low whistle, the other with exchangable heads. It’s the latter one which gives the game away: you cannot play the ellisions podsible on a traverse flute. The gody’d was the same, it just had two heads, exchangeable.
What do I mean by elisions? Until the Roli slide keyboard came along, it was the only instrument in the Western hemisphere where you could, with practice, gliss from note to note over the entire range in it’s basic voice, because the finger holes are so big you can gently move from closed to open by slow degrees. What we actually did, was Paddy, having first courteously asked to sit in, glanced across and asked, “D’you play?” Well, in such company, a moot point, but I had learned against Jimmy Galway recordings, so I couldn’t turn such a request down, really, could I? I mean, a masterclass with Paddy? Well…
So we started with some basics, which I graced, so he knew that perhaps I could. Then some tunes, and again, so we were away, him on first, me on seconds. Don’t ask me where we went, we met a washerwoman along the way, for sure. He was enjoying himself, because I wasn’t playing book seconds, but by ear, so he could really experiment. So my answer to why is because they lay down the dancefloor for other instruments to perform from. They’re more interesting than a drone, but live in the same range, and are more amenable to creating a beat, both in rhythmic and in acoustic terms.
Yes, they were invented in 1971 by Finbar Furey, to replace a Brazilian whistle someone had crushed, but he was heavily into the Brazilian fad of the day, and I don’t think there are any recordings that early of him playing Irish on them. His recordings are later, after Riverdance and LotR.
In the 30 or so years that Irish flute was my main instrument I didn’t play Low Whistle much. Coming from the flute, I was more conscious of the things the Low Whistles couldn’t do, rather than the things they could do. In truth I didn’t give the instrument much of a chance.
The one thing the Low Whistle was handy for was the times in the band when I had to play right after playing Highland pipes. I know there are people who can do it, but for me it seemed like the mouthpiece of the Highland pipes was devilishly effective at getting all the muscles of my embouchure out of whack.
But eventually the flute took too much of a physical toll on me, my neck and wrists in too much pain, and I sold off my flutes and went on a Low D Whistle buying spree. (You can buy ten top-notch Low Whistles for the price of one good flute.)
The whistle, the clarinet, bagpipes, and most other woodwinds have the head, shoulders, arms, wrists, and hands in ergonomic alignment while the flute requires a compromise due to the head and hands needing to be at around a 90 degree angle.
So the flutes had to go but I’ve continued to play the pipes and whistles in complete comfort.
“… and I don’t think there are any recordings that early of him playing Irish on them. His recordings are later, after Riverdance and LotR”.
Not as early as 1971, but these recordings were made years before “Riverdance” :
https://youtu.be/DtluiWzfbNc
I don’t. I tried, but I find it ergonomically really difficult to hold and play. I have a couple, and I just took them out and tried again, and yes, unpleasant and actually kind of painful to play. I have an M+E that i really should sell. Or maybe get one of those susato thumbrests
Whole different feel. A friend of mine says mine sounds like a kaval, and he loves Balkan stuff. It does indeed work well for the less complex Bulgarian tunes
I originally was going to learn irish flute and play whistle for keys that werent low D. But then I caved and got a low D whistle because my whistle paying got 100x better than my flute playing haha. Embouchure is just hard. So now I never pick up my flute. Which is wood and not worth maintaining if I play it so I just dont touch it. I’ve been tempted to get a used Delrin Copley or Seery or something so I can get back into it. To me they just give different sounds. Irish flute doesnt sound like a low whistle.
As a bonus flute avoids some of the cons of whistles. Whistles are hard to get the right balance of characteristics. Flute the player is a bit more in control, if they have the skill to control it. At least I think, my control was never good enough lol.
Also its just something different. I like playing multiple instruments, it just feels good. Flute and whistle are more similar than most so the carry over from playing one is decent.
How fun to discover that the crushed whistle was Brazilian since I am Brazilian too hahahah Makes me feel somehow more connected with the origins of the instrument I love so much. I wanted to play the low whistle for some time and last year I finally was able to buy a Chieftain V5 
Not that it matters a lot but don’t make too much of it. The standard version of the origins story of the damaged whistle is usually a slightly different one:
Bernard Overton, an English flute maker and jazz musician, is the original creator of the modern low whistle as we know it today.
His first model was built to replace an Indian bamboo whistle which belonged to none other than the legendary Finbar Furey (member of famous Irish folk group, The Fureys). Finbar’s poor whistle was destroyed when someone sat on it in a pub in Coventry, England.