There's a simple answer to this,

there are two different whistles being talked about here.... personally I think the one in the Celine Dion song.. is just your ordinary standard tin whistle (not in D)..... I haven't got a recording here. but I tend to agree with the people above who say Bb or C.

But in the movie itself...... there is a low D being played... (don't ask me to quote scenes)

Colin.... my generations have been easy to break in.. always played right from the word go....... and I've never had to tune them (unlike the sweetones), although I have cleaned them using a method similar to your one above..... if susato came up with a decent mouthpiece I'd probably play susato.........

slainte

alison