Tin Whistle player here…
One of the great things about the tin whistle is that you don’t have to spend big money to get a good one. One of the best around is the Clarke whistle.
They make ones with a plastic mouthpiece (called the Clarke Sweetone), but the one I really like is the original whistle, which is solid metal and has a wooden plug. There are some professional whistle players who play with this whistle. I highly recommend you find a starter kit that has this whistle in it. Or you can just get the whistle alone and learn on the web (links below).
They’re only around $10-$15, probably with a book to teach you the basics, and they sound wonderful. They have a nice mellow smooth tone to them. I have a $150 wooden whistle like this from Sweetheart Whistles, and I actually enjoy playing the Clarke more.
There are differences between various whistles that are worth mentioning. Some require more air flow than others. Some are very loud, and are intended to be played with a band, while others are quiet. In the case of the $150 whistle I have, it’s made to play with a band, so it has a tunable barrel so you can tune yourself to the band, and it’s quite loud.
Start with a whistle in D, as that’s the most common key for whistle music. Then you can add more whistles later (being very cheap, it’s fun to collect them).
Here are some good Tin Whistle links for you to check out:
Brother Steve’s Tin Whistle Pages
Chiff and Fipple
Chiff and Fipple has a guide to inexpensive whistles that should be useful for you: Guide to Inexpensive Whistles. They also have an active message board where you can ask questions.
The Sessioneer
Whistle Tutor
Another great thing about the tin whistle is that so much of the traditional music played on it is in the public domain, so you can find lots of it on the internet. The links above will have lots of tutorials using authentic irish jigs and airs, including MP3s of the songs so you can hear what they sound like when played correctly. It’s a lot of fun just surfing around and picking up songs here and there and trying to match the recordings.
Hope this is useful for you.