If you play traditional dance music, the likelihood is that you will have started on a D whistle. In this case the simplest course is to transpose the target tune into whichever key playable on a D whistle (D, G, Emin etc) gives the best fit. Then play the tune on whichever pitch of whistle gets you back to the original key, but using the fingering you would uee for a D whistle.

For instance, you want to play a tune/song that's in F. It has a range of C > AA. Transpose it to G, and then learn it in G on a D whistle. You are playing it a tone sharper than original, but the written key and the actual fingering will both be familiar. Replace the D whistle with a C whistle, but continue with the fingering used for the D whistle, and you will now be playing in pitch.

A simpler alternative, particularly if you have a good ear, is to learn and play all tunes in tonic sol-fa. This way the fingering pattern will be constant, and again the key you play in will be determined by the pitchy of the whistle.

Both these methods avoid the complexity of remembering that (for instance) covering the top two holes gives A on a D whistle, D on a G whistle, B flat on a E flat whistle, etc, etc - and similar confusions for every other note you finger.