You shouldn't need a licence for any music or song which is in the public domain (i.e. genuinely traditional, not just assumed to be). And copyright on printed books of traditional songs (as mentioned on another thread) does not necessarily extend to the text of the jong, just the book itself. There are sites which contain lists of songs in the public domain in the USA, (most music published before 1923) but this cannot be relied upon for the UK, as copyright laws are different.