TheSession.Org
This web site has a ton of tunes in staff notation and ABC format, along with commentary and historical notes by Irish musicians.
Mike Simpson's Tin Whistle Guide
This is a useful guide and tutorial for the Irish tin whistle (download, unzip and open index.html using your browser).
Brad Hurley's Irish Flute Guide
The web's most complete guide to Irish flute, including a directory of flute makers around the world.
Field Guide to the Irish Music Session
Information that anyone playing in or listening to an Irish session needs to know.
The Companion to Irish Traditional Music
This encyclopedia-like volume contains a ton of information on instruments, history, and legendary players past and present.
O'Neill's Music of Ireland (1903 version)
Known as the "Bible" of traditional tunes, this famous compilation has 1,850 tunes collected by Capt. Francis O'Neill and James O'Neill in Chicago.
O'Neill's Music of Ireland (revised version)
This version was revised by Miles Krassen.