The intent is to represent the original O'Neill's publication as nearly as can be done with ABC music notation. Many of the tunes here are played differently these days, which is normal for a living tradition. In some cases, modern music notation would do some things differently than was standard in the early 1900's. This is especially true for key signatures, many of which are "wrong" by modern standards. But none of this diminishes the value of O'Neill's book as a historic reference.
Lovers of traditional Irish music will want O'Neill's in book form. A facsimile edition is now published by Mel Bay. You might get it a bit cheaper from some of the online book sellers, but if you don't mind the few extra dollars, buying directly from Mel Bay would be a nice gesture to a publisher that produces quality copies of such historic music collections. Here are some places to look:
Mel Bay amazon.com Barnes & Noble
Everyone with a copy of O'Neill's is invited to compare the ABC here with the original. As of January, 2000, we have at least one transcription of every tune, and two of some. However, not all have been proofread, and there are probably still some typos. If you find any discrepancies, send me mail.
The original O'Neill's Music of Ireland has been out of copyright for many years now. This collection of ABC transcriptions is copyrighted 1997-2000 by the contributors to the project. You are welcome to download anything here and use it, as long as you give due credit to the transcribers, and take credit for any corrections or enhancements that you might make. If you want to mirror the entire collection, please get in touch so we can arrange to keep all the copies up to date.