With the best intentions in the world as people do their best to preserve these wonderful tunes, errors creep in and are passed on. Often the differences are simply variations because tunes were passed along by ear. It can take a lot of research using many sources to make sure you've got what you set out to find.

• Ceolas.org
  largest online collection of info on celtic music, and 100's of links
• Henrik Norbeck's ABC Tunes
  1661 Irish tunes (in ABC & Midi format)
• Irish Tune.info
  Irish Traditional Music Tune Index - Alan Ng's Tunography
  extensive; well-researched; accurate; Irish session and dance music
• The Fiddler's Companion
  Andrew Kuntz - songs & tunes in ABC format; plus history, discography, sources, and often several variations
• County Champaign Irish Tune Collections Online
  Jerome Colburn - closest to session music played hereabouts
• Comhaltas Ceoltóirí Éireann
  international organization dedicated to preservation of Irish traditional music
• JC's Online Music Books
  A number of historic tune collections transcribed into ABC notation
• The Reverend's ABC Tune Search
  ABC Tune Search for Irish traditional music, by Reverend Pete.
• Richard Robinson's Tunebook
  a collection of traditional tunes, and new tunes in traditional styles - Scots tunes, Irish tunes, Scandinavian, English, French, Balkan and more
• Irish Traditional Music Archive
  Ireland's national reference archive and resource centre for the traditional song, instrumental music and dance of Ireland
• Donagh MacDonagh Song Collection
  song lyrics not commonly available elsewhere - collected from radio listeners in Ireland in the 1940's and '50s
• Dennis Gormley's "Grab Bag of Tunes"
  well-known in Philly, Dennis Gormley is a member of "McDermott's Handy"
• Folk Music of England, Scotland, Ireland, Wales & America
  Lesley Nelson-Burns' (aka the Contemplator)'s Folk Music Site
• McKenna's Irish Music site
  bands, musicians, instruments, lyrics, tune books, sheet music, etc. by Hans Jørgen "mckenna" Jensen
• Tune Collections in ABC notation
  Chris Walshaw - plus learning ABC
• Tune Collections on the Internet
  Impressive collection of links to other collections
• The Session.org
  by "Jeremy" - not fact-checked:  a lot of mis-information in with the good
• Sing Out!
  Bethlehem, PA - dedicated to the preservation of traditional folk music
• Villanova University Library's Audio Collection
  Digital archive of Philidelphia Ceili Group's annual festivals and other events available to the public in digital form as part of the Villanova University Library's audio collection. At present (as of February 2016), audio from the 1977, 1978, 1979, 1989, and 1996 annual festivals are available. Recordings from every festival from 1977 to 1996 will be added. Project began in the Fall 2012 and is expected to take seven years to complete.
  Now posting recordings of events hosted by the Philadelphia Ceili Group, including: concerts, lectures, poetry, stories, etc.