Not being able to hear yourself in a session isn't limited to whistles.

When I play mandolin, I run into the very same thing--can't hear myself, and must struggle against an inclination to "force" it, playing harder and louder (and sounding worse as a result).

You should ask amongst the listeners if you can be heard, and not rely solely on your own ears. I'm told that the mando, being a different "voice," cuts through quite well: it's only me who can't hear (maybe 'cause I'm sitting behind it?). The whistle may present the same situation--in our local sessions, I can always hear the whistler. and we've got the whole gamut--accordion, pipes, fiddles, the odd bouzouki and harp ...

If the consensus really is that you're inaudible, get a louder whistle. I have a three-barrel Susato set (D, C, Bb) that I rarely play precisely because it _is_ so darn loud. It wasn't too expensive (@ 10 dollars per barrel) and might solve your problem.

Why limit yourself to a subset of tunes? If your whistle isn't loud enough, get a louder whistle, I say.

Jenny