For a quick breakdown of the most comfortable keys to play in on each whistle, see the list below:

Playing in various keys on the tin whistle couldn’t be easier. There’s no need to relearn tunes or fingerings – you can buy another whistle in a new key.

The tin whistle is actually a transposing instrument. That means the notes it can play are dictated by the key your whistle is in. What does this mean?

When you play the lowest note on a D whistle – six fingers down, with one covering each hole – it will sound as a D. The same fingering on a C whistle however will produce a C instead.

The easiest keys to play in on any whistle without the use of cross-fingering or half-covering tone holes are the tonic (the key the whistle is made in) and the subdominant (a fourth above the tonic), in addition to their relative natural minor scales.

On a D whistle, for example, you can comfortably play in D major, G major, B minor (natural minor) and E minor (natural minor).

You can also play in A major  (the dominant – a fifth above the tonic) by half covering the G tone hole to produce a G# accidentally (remember, this is only recommended for occasional passing notes).

Based on: The Complete Guide To The Irish Tin Whistle

This is why I have all the whistle keys mentioned above :).