ValeryA good basic start would be two books called "Irish Street Ballads" and "More Irish Street Ballads" They wer bublished about 40 yrears ago but reprinted as a paperback a few years ago. Lots of songs, tunes and notes.
Thre was also a set of small books by a publisher called Gilbert Dalton called "SOngs of COrk", "SOngs of Dublin" etc. Again, out of print but reissued.
You can probably contact a suitable bookseller through the Net.
The Digital Tradition itself is a great source - often showing up the connections between versions of a song as it wanders the world.
Mind you, you have to remember that many "Irish" or celtic songs are in fact English or Scots - its just that we Irish kept singing them longer! Barry's message makes the same point, really.
"Twisting the Rope" is a translation of a Gaelic song called "Casadh an tSugain". I'll find a text if I can.
Regards
Martin