The 1850 collection was the first known to have been transcribed to ABC notation, making it very available to computer software as well as humans. The work was done by a group of about 20 people from 1996-1999, and has been online at several sites since then. The original collection was organized mostly for people using search sites, and for those that wanted to download the entire collection in ABC form. This directory is a "reorg" aimed at making it more useful to people using computers, especially portable electronic gadgets, rather than paper.
"The Dance Music of Ireland" was published in 1907. It's called "O'Neill's 1001" for its number of tunes. There's a large overlap of tunes with "1850", but "1001" concentrates on the versions used in dancing (generally longways reels and jigs, and Irish step dancing).
There might be some confusion about just what is here. Some tunes were transcribed by more than one person. Other tunes are in 2 or 3 different versions in the original book, usually indicated by 1st, 2nd, etc. after the title. There are also several cases of more than one tune having the same title. So the confusion is understandable, and can be only partly clarified. Fairly often, the easiest way to indicate multiple tunes or versions with the same titles is to append the tune number to the title in the file name. This may be accompanied by other info such as "2nd", "3rd" (for multiple settings of the same tune), or a single-digit number for "condensed" versions that require less screen space due to the use of more alternate endings and/or the P: part-order notation for complex ordering of labelled sections of a tune.
The row of capital letters take you to the section of titles that start with that letter.
This list of tunes is alphabetical, based on a tune's English filename (omitting an initial "The" or "A" or "An"). Basicaly, all the non-letters and initial articles are stripped from the title, and the result is what is alphabetized. The software may show a surprising order in a few cases, as computer software sometimes has odd ideas about how to alphabetize things. so you might look around a bit if you don't see a particular name. There are also some odd spellings in a few tune titles.
The columns at the left are buttons that return a tune in several formats. The "abc" link merely returns the ABC source file. If there's a link in the "img" column, it returns a page formatted for printing. The others don't add any borders, and work better on a small screen, but may print oddly (depending on your gadget's software).
If you have question or suggestions, send me email.