well I don't know how these women are learning these songs but if you are anything like me a few good recordings are very helpful...it is hard to learn a melody from some of the the books out there. I would like to recommend some of the singers whose work I have learned a lot from, whether I sing something in their arrangement, or get the basic song from them and use another version of lyrics or slightly different melody I have heard elsewhere, etc. My specialty seems to be doing sad, obscure ballads a cappella, so hopefully this will be useful since that sounds like what you are looking for...

Mairead ni Dhomnail (No Dowry, or Traveller's Prayer where she sings with The Voice Squad)
June Tabor (Airs) Child Ballads (often you get several versions right in a row! Not often the most lyrical singing but lots of good sean nos style)
Karan Casey (with Solas or without)
Capercaille (Karen Matheson sings lots of Scots Gaelic songs)
Triona ni Dhomnail (sings with the Donal Lunny Band now, I think)
Loreena McKennitt (early stuff is more traditional, has nice harp accompaniment)
Altan
Anam
Niamh Parsons

hope this helps...

peg