I live in Japan so can buy mirin easily, but I usually use brown sugar to replace it in recipes that call for soy sauce, mirin and sugar. Since it is fermented I should think it would take forever to make. Brown sugar (medium dark) gives you the same taste in the finished product. Just use the amount of sugar recommended as brown rather than white. Saves money too.
Cantate
By
09/28/2007
Here is a link I found for making mirin on an industrial scale.