Mirin is a Japanese condiment which contains about 14% alcohol. To make mirin, steamed mochi-gome (glutinous rice), kome-koji (cultured rice), and shochu (distilled alcoholic beverage) are mixed and fermented for about 2 months. Mirin produced this way is called hon-mirin, as distinguished from mirin-style condiments (mirin-fu chomiryo) which is made to resemble the flavor of mirin. Mirin-style condiments contain less than 1% alcohol, and they are usually cheaper than hon-mirin. Well-known Japanese brands for mirin are Takara and Mitsukan.
Mirin is a clear, gold liquid. It adds a mild sweetness and nice aroma to many Japanese dishes. Especially, it helps mask the smell of fish and seafood. Mirin also adds luster to ingredients and is a key ingredient in teriyaki sauce.
The use of mirin is said to have begun over 400 years ago. Although it was used for drinking in the beginning, it has been used just for cooking since it became thicker and sweeter.
You can use sake and sugar for mirin if you need. The basic ratio of sake and sugar is 3 to 1. It's good to use 1 Tbsp of sake and 1 tsp of sugar for 1 Tbsp of mirin. Adjust the amount of sugar, depending on your preference.