But once a person does know, it IS possible to learn new pieces from written music

You see? Both you and Snail always have to qualify your claims that sheet music is OK by saying that you "need to know" first. Thanks for making my point, both of you.

And I'm only saying for traditional Irish music. That's what I happen to know something about. Not songs either. I suspect it's exactly the same for other styles of traditional folk music though I wouldn't ask to be quoted on it.