I was googling away looking for a low whistle and when I searched for Kerrys I get a link to their website, all good. But when I look on say Amazon it throws up Chieftain and Thunderbird, both under Kerry. And who makes Generation?
Here you can see those various names, and more, used for Phil Hardy’s various whistles
http://kerrywhistles.com/shop/
On that page are
Kerry
Chieftain
Thunderbird
Optima
V5
It’s more complicated that even that. Bernard Overton is credited with inventing the all-alloy (aluminium) whistles back in the 1970s and at one point three people were making Overton whistles: Bernard Overton, Phil Hardy, and Colin Goldie.
Phil and Colin were authorised by Bernard to make and sell whistles under the Overton name.
(I have a Low Whistle stamped OVERTON which is signed by Colin Goldie inside the bell.)
So there are early Phil Hardy whistles in the Overton style, but later he began making other styles, under his own brands.
Here’s a video Phil made explaining the history of the Overton whistles
https://www.youtube.com/watch?v=ewzpB2zDFT8&t=504s
He used to have a great video about all of his various Kerry Chieftain etc whistles but I can’t find it now.
This one maybe?
https://www.youtube.com/watch?v=V2xPomi-snU
Not really up to date however as he recently made some changes to the Thunderbird low D which now has a higher backpressure and plays more like a V5 regarding the amount of air you need (according to Phil).
How long has the low whistle been around? I presume its a relatively modern phenomenon as originally the tin whistle was a small, soprano instrument.
1970s, I believe. At least In irish music. There were probably low whistles before that in other genres
Since the early to mid 19th century. (Also depends somewhat on what you mean by “low”.)
If we include the old English flageolets (wood whistles with one to three or four keys), they made those down to low G early in the century. I believe Clarke made some low Gs in the mid 19th century. And there were certainly low G cylindrical whistles in the last half of the century.
I think anything lower than that (tenor (low D) & bass (really low G) whistles) is an invention of the 20th century.
And what is the lowest, and thus largest whistle? In the world of the recorder they get very large, bass, contra bass, sub contra bass, octobass etc. Is there such a size in a whistle?
I’ve seen a video of a bass ‘A’ - but that’s not a common whistle, as far as I know. 
I think the lowest would be determined by the length of your arms. 
Unless it had keys…
I have a bass in G (Shearwater). I think G is the absolute lowest commonly made, and I think only a couple makers venture below bass A.
Our own Daniel Binghamon makes or used to make a bass in D, and yes it does have keywork throughout!
He also makes (or made) a wee d whistle (an octave higher than the high d whistle), with a clever slide mechanism that makes for an ideal introduction into the wonderful world of penny whistles for the trombonists. The only thing higher than that is a bird flageolet, and at least the ones I’ve seen are in wee g, smaller than Daniel’s little whistles.
If you mean the Low D whistle, on that video Phil states that Bernard Overton made his first ones around 1970, as I understand for Davey Spillane.
My first Irish music mentor, back in the late 1970s, played an Overton Low D as his primary instrument.
Evidently at one point Generation made “mezzo” G whistles.
Uilleann piper and pipemaker Joe McKenna tells the story of buying several red plastic Low Whistle tops in a junk shop in London in the 1970s. They were probably these Generation “mezzo” G tops, which helps date them, because Generation only switched to injection-moulded plastic tops what, around 1960? (There was a big craze for injection moulded plastic things in the 1950s.)
Joe then made brass tubing bodies for some of these and toured with them. I saw Joe here in California in the late 1970s and early 1980s playing at least one of his self-made Low Whistles. Seems to me that he had one bigger than a G, perhaps an F, but I can’t exactly remember.
Mary Bergin played one of these, in G, on her iconic Feadoga Stain album.