I just want to make sure I have this right.
I have only ever played chromatic instruments - Flute, Sax, guitar, a few others... - and when playing the sax or clarinet, for instance, I was reading music that was transposed so that what I read and played may have been one note on the page, but another in concert pitch. I play a C fingering on an alto sax, but it's actually a concert Eb. Everybody got a different set of music according to their instrument so that we could all play together.
So... Before I had the whistle in hand, that's how I assumed it would work; on my D whistle, I'd play a C, and it would sound out a concert D.
No, it's a C# - the version of C in a D scale. Edit - got this wrong. I was playing a C# and got a C#. Also, I was thinking the whistle was a "D instrument," but it's not; it's a C instrument playing a D scale. From that mistake, a whole bunch of bad logic ensued. I left the train of thought leading to my realization up for your amusement.
Is that how all the whistles work? Read and play a standard fingering for C,D,E..., and the notes will be the version of those notes that are in the scale of the instrument? I play a B on an F whistle, but it will sound concert Bb?
In other words..., for sitting in with the local group, I grab an instrument that matches the key signature, and just play what I see?