If’n ya want that . . . play a D whistle.
But that’s the whole point of whistles-as-transposing instruments. Get a whistle in the appropriate key and play it as a D whistle. Saves having to change fingerings when you change keys.
About the only time that I break that rule is when I want to play a tune in D with notes below the whistle’s low D. Then I get an A whistle. (The “G” fingerings used on an A whistle give the key of D with three lower notes available - C# B A)
For most IrTrad, though, the tune doesn’t go below D, anyway - so you use a C whistle to play in C, a Bb to play in Bb, etc. Clear?