In a recent survey over at The Session Website of all the traditional tunes on the site, the most popular key was D with 1191 in that key. Here's the list that was compiled.
There were 4 tunes in the key of Eb
There were 33 in Bb
There were 99 in F
There were 191 in C
There were 738 in G
There were 1191 in D
There were 719 in A
There 145 in E
Generally speaking a C whistle can be used to play tunes / song in the key of C, F, Gm and Dm.
A D whistle can be used for most keys when you half cover a couple of holes.
Most of the sessions I attend the whistle players bring along several whistles in different keys. This saves them from covering half holes. For example when someone is about to start a tune or song it's customary to tell the other musicians what key they are going to play in. This gives the other players notice so they can change whistle.
There are over 100 whistle songs on this site that have youtube videos to show you which notes I'm playing. I generally use a D whistle. If you want to change the key of any of the songs you simple choose a different whistle and play the very same notes that I'm playing on my D whistle. That's probably the easiest way of changing keys on the whistle.
Irish folk song lyrics, chords and a whole lot more