You can play any tune on a whistle, but

1) It has to be in the main key of the whistle, or one of the related Major or Minor keys - you may need to transpose.

2) Since the whistle is naturally a diatonic instrument, only the 8 notes of the Major Scale (D Major for a D Whistle, etc) are simply available. Other notes of the 12 chromatic notes may be achieved by cross-fingering or half-holing, but it takes more skill (and practice!) to achieve that. So while the easiest music to play, especially for beginners, is say in D or G Major or E Minor on a D Whistle, more advanced players can often handle other relative scales.

3) Simpler tunes will be easier for beginners, so just try anything, especially whatever you happen to have in your head at the time....

4) Style of Music - Anything - not just Celtic/Irish, for goodness sake!

Robin