My absolutely favourite ABC site is
http://www.thesession.org/ where you can search for thousands of irish / celtic tunes. Besides ABC it also has sheet music and more information about almost every tune, eg who recorded them, on which CDs they are to find etc. A very nice feature is that you can save your own tunebook (after registering only) , that you can sort by name, style, key etc, and after that download as one ABC file, very comfortable!!!
One great advantage of ABC files is that you have the tune in its basic origin, which makes learnin the tunes quite simple. If you try to learn tunes from a CD it is often pretty hard because every musician plays decorations fitting to his instrument and his taste, so that sometimes it is almost impossble to recognise the basic melody. If you have the ABC you can learn the tune, and then make it fit to your own taste.
Another very nice feature of ABC Navigator is that you can arrange tunes to sets, very helpful because irish tunes are usually played in sets of 2, 3 or more tunes. This feature is also good for practicing key changes or harmonica changes if you prefere to play harmonicas in different keys.
ABC Navigator plays the tunes with the patches of your choice, I prefere flute, to practice tunes, and you can choose the playback speed, excellent for practicing and learning tunes. Besides the ABC, which is difficult to read for me, it also shows the tunes as sheet music.
So if you like celtic tunes ABC Navigator or similar programs and excellent sites like thesession are very helpful.
Have fun, and all the best from germany,
Dirk