But when Cian was placed in the glass his name was stricken from the Keltar books and much of the sacred lore was lost, because the room with all the lore was sealed and was opened again till the 1600s. This is why in "spell..." that they were unsure if Cian was a real person or some myth.
But someone was responsible for contiuning the teaching of the Druid Arts, because the Keltar twins were taught years later.
Also, Aoibeal said in "Spell..." that she had saved the extinction of the MacKeltar a couple of times (dont know the exact number if stated).