[quote=@Beloss]
Arturia knows who Diarmuid and Cuchulain are, and they her, even though that should technically be impossible. It is information that the grail has given her directly similar to how she knows modern language and most customs.
[/quote]

And here was I being a fool all this time and thinking that it was because King Arthur was a literate person (given that he was a noble) that had a Celtic Druid older than dirt as teacher and thereby was bound to know, y'kow? Legends from the very archipelago she was born in...

Because it's not like Ireland is so very far from Corwall and the rest of the Brittish islands.

Well, such is life. You always learn new things everyday.