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.
This video is a perfect review for the beginning whistle player. As of 5/28/2023, it was operational.
https://www.youtube.com/watch?v=g2PNC49fyMI
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.
Michael Eskin's Interactive King Street Book of Irish tunes is one of the best practice and learning resources that I have found for Celtic music.