Wikipedia’s list of musicans who play left-handed
Is playing “right hand on top/left hand on bottom” considered “playing left-handed”?
I don’t know why that list should matter much. I’d question whether some of the names on that list are actually what should be calling musicians. 
Mary Bergin’s wiki page says she plays left handed. How’s she sign her name?
I don’t think the whistle is an instrument who’s design favors one hand over another. Prettty much a toss up really. It’s not like a keyed flute where the mechanics somewhat dictate one hand in a particular position. It matters very little in my mind whether the right or left hand is on top with the whistle. More of a player preference to me. I’ve seen enough kids, naturally right handed kids, place the right hand up top to imitate a mirror image what a teacher does. No harm in any case - 'til they pick up a keyed flute.
What about Seamus Egan?
Feadoggie
Is playing “right hand on top/left hand on bottom” considered “playing left-handed”?
In general, yes.
But there are quite a few who play hands reversed while not being true citeogs. Which can happen when being taught by a left handed player and following their example. It’s not really a problem but I have had a number of young students who had learned the whistle left handed (while being right handed) who then came to me to learn the pipes. Continuing on left-handed then may become a problem when it’s time to play the regulators.
Cross-posted.
I’d say no. Unlike the other instruments in that list, which involve a division of handed distinctive action (e.g. fretting vs. picking), there’s nothing about whistle to enforce or favor either hand on top, or encourage a left-handed individual to reverse the hands.
And while RH on top is casually and conveniently called Left-Handed position, you’ll find many otherwise normally right-handed people playing whistle “left-handed”, because that’s the way they learned. If Mary is actually normally left-handed (and I’m sure someone here knows), then I suppose she could arguably be described as playing left-handed. But it’s not a very meaningful distinction on whistle, except as a raw description, and not necessarily list-worthy IMO.
How about Seamus Egan, who plays whistle vs. flute with opposite hand positions? 
I admit, though, practically speaking I encourage people to learn whistle with LH on top - especially if they’re also interested in exploring other keyed and orchestral winds which are definitely not handedly symmetrical or neutral, and for which right-handed instruments are much more readily available.
Also cross-posted with both the above.
Guessing she might not be because the first volume of her tutor says:
Though FWIW I describe myself as playing guitar and bass left-handed despite being a natural right-hander.
I do know one of the people on that Wiki list: my old friend and high school rock bandmate Jimmy Haslip. He is left-handed IRL, and he plays a normally strung bass guitar in the flipped around left-handed position. The reason being, in his case, that a normal right-handed bass is what came his way to learn on, and he couldn’t really afford a custom-configured instrument back then when we started in the late 60s.
And you have very good reasons for doing so.
Yep, I remember (having fortuitously tripped over this previous discussion while revisiting some old posts yesterday)…
https://forums.chiffandfipple.com/t/just-curious-playing-same-string-inst-right-left-handed/77196/5
Which isn’t the only time I’ve talked about it here, though you weren’t part of this one…
https://forums.chiffandfipple.com/t/on-left-handed-keyless-flutes/81273/7
Think I read somewhere that Mark Knopfler’s a natural left-hander, but can’t confirm!
Yes, I think that is the case. As I understand it he started out on violin and was taught to play in the generally accepted position with the bow in the right hand and the left hand on the neck. When he took up guitar he pretty much stayed the course, so to speak.
I’m right-handed in everything else, but when I picked up the whistle I played it left-handed because I didn’t know any better - had no teacher, I just learned by ear from Sean O’ Riada records. Many years later when I took up the pipes, I had to decide whether to get a left-handed set or to re-learn the whistle right-handed. I decided to re-learn the whistle. It was frustrating for a while but I’ve successfully made the switch now and don’t even think about it any more.
I agree there’s no obvious correlation between hand dominance and whistle-playing preferences. I’m left handed and play with my left above my right (but would prefer to play the guitar left-handed, though I’ve also learned right-handed), and my son is right handed and plays the whistle with his right above his left – I don’t see the rhyme or reason (he has largely picked it up on his own, and not from watching me). Of course, neither of us belong on that list, just sayin…
Briant