Chiff and Fipple
The Tin Whistle Internet Experience. The best site on planet earth to get information about whistles.  Try the forums.  Lots of friendly folks that love to answer questions about all kinds of music.

The Session
The place to go to find sheet and tabs for tunes and to locate your local session.  Some of this info is really old.

Home Spun Tapes
Check out the excellent tutorial books by Cathal McConnel and others.  Excellent for learning lots of instruments.

Ceolas houses the largest online collection of information on Celtic music, and has links to hundreds of related sites.

Whistle/Flute Calculator
Use this calculator to create your own whistle or flute.
This is a mostly Gaelic TV station in Ireland that is online, too.  It has some great music from time to time as well as other things I find interesting.

PJ (TinWhistler) on youtube
These guys rock.  Mostly guitar and low whistle.  Other stuff too.

Live Trad
Lots and lots of great traditional Irish music videos.  I go here a lot.  :)