This is an excellent tin whistle site created by the talented instructor Stephen Jones that includes a seriese of basic online tin whistle lessons. Working through his online lessons will give you a solid grasp of the basics. His lessons on ornamentation are exceptional, and, in my humble opinion, Steve's coverage of ornamentation and the essence of jigs is superior to any of the printed book/CD tutorials available on the market today. Oh, and its free. It is clear from the layout of his site that Steve has years of experience as an effective teacher of music. His simple explanations of the concepts, use of numerous helpful sound clips in his lesson plans, excellent selection of sample tunes and methodical progression from simple concepts to more complex aspects of tunes and technique are all elements of the success of this site, but the most valuable thing I see about this site is that Steve brings his own unique concepts, ideas and perspectives to the lessons that I find to be very effective. Other than Chiff and Fipple, Brother Steve's site is arguably one of the most valuable sites on the Web from the perspective of a beginning whistler.
The Whistle Workshop
Dave Auty has created a great site for beginning tin whistle players. This site starts from the beginning, with fingering charts, and excellent coverage of essential basics that other tutorials gloss over, such as scales and arpeggios. He also covers phrasing and articulation, as well as MP3s of many jigs and reels with accompanying musical notation to illustrate his points regarding phrasing and ornamentation. Dave's Whistle Workshop compliments Brother Steve very nicely, and collectively I see these two sites as being an excellent and fairly complete resource for beginners.
Sessioneer
The Whistle Tutor