Tinwhistle websites seem to come and go but those listed below are currently free to use with no log-in required.
www.clarketinwhistle.com This link is an excellent website, which introduces a brief history of the whistle and is beautifully narrated by James Galway. There is also a good explanation of the different sounds whistles from various metals will produce. Sound files have been provided so you can hear the difference.
www.musicbypatty.com This is my website where I have posted notes from all of my classes. Just go to LESSONS on the right side of the homepage and click on it to produce a drop down menu. Select the appropriate class and print any desired lessons.