Afraid I can't remember very clearly about the book. Not living in the country anymore but it should be available by now. In English rather than in Irish. One of the UCC academics I think. Sorry about that.

I doubt if cailín or caileag derive from cailleach (the diminutive would be cailleach(a)ín). Cailín is diminutive of caile which meant a servant girl at first (buachaill for boy also meant a cowherd originally - the Court of Justice, Nike and Nestlé are currently investigating child labour conditions in early Ireland!!). Apart from the difference between -l- and -ll- cailín is one of those words whose grammatical gender differs from the sense, being always masculine, while cailleach is always feminine - there is a resemblence but possibly no direct derivation one from the other.

Sorry again about the lack of detail about the book. If it flashes across the brain I'll post it.