Captain Francis O'Neill's collections are probably the most influential in the history of Irish music.
"O'Neill's 1001" was published in 1907 and contains the dance tunes from "O'Neill's 1850" (published in 1903) as well as some additional tunes. For some tunes the settings in the 1001 are quite different from the settings in the 1850.
This collection was originally transcribed in ABC format by Frank Nordberg and published on musicaviva.com, which does not exist any longer.
John Chambers hosted a mirror on his site, where he also made some corrections.
Then I have made some more corrections before publishing here. See comment at top of ABC file for details.
So far I only have tunes 1-365 here.