Michael, if it still isn't clear what the folks are saying about keys, try this.
Whistles come in different keys (different lengths and bores). But you finger each one the same. You are holding a D whistle and the fiddle player is in C? Set it down and pick up your C whistle.
Playing a tune in a key incompatible with your whistle will mean fingering a lot of half holes. It's easier to change whistles, I find.
Though, as CraigS says above, you can also make a D whistle into a G with one half hole or a different fingering. You move your octave or tonic note up to the fourth note/third hole (instead of the first note/all closed holes).
That is to say, there are a couple of keys related to D that are easy to play in. (G, Em) After that, change whistles.
Did that help?
Blessings,
Barbara