I rode my bicycle past Jomyo-in Temple hundreds of times on my way to work, never suspecting that it might be worth visiting until last year they started doing construction on it, and I got a look inside because one of the walls was torn down. It’s actually pretty interesting because its filled wall-to-wall with thousands of Jizo sculptures.
Before the Meiji Restoration, all of Ueno Park and a lot of it’s surroundings were one huge temple called Kan’ei-ji, and Jomyo-in was one of its 36 sub-temples. Kan’ei-ji was closely associated with the Tokugawa Shoguns, and Jomyo-in is named for the mother of the fourth Tokugawa Shogun, Tokugawa Ietsuna.
The temple was renamed Jomyo-in in 1723. The front gate is said to date back from this time.
The jizo thing was started by a monk called Myoun, who became the chief priest of the temple in 1876. He was originally from Osaka, and at the age of 25, while living as a hermit at a temple in Nikko, he came have great faith in Jizo. He started out with the idea of making a thousand jizo statues, but when they were finished, he started thinking big and decided to go for 84,000. The temple and some sites that I checked seem to indicate that there really are 84,000 jizo statues there, but there clearly aren’t.
There’s a really cool 360 degree panoramic photo of the temple here: http://www.360cities.net/image/jomyoin#695.86,-9.07,110.0
The temple is right next to the entrance of the Yanaka Cemetery.
There’s a very good map and detailed access information on this PDF: http://www.yes-tokio.es/pictures/fichas%20zonas/yanaka.pdf
Here is the temple’s official homepage, in really difficult to read Japanese: http://www.tendaitokyo.jp/jiinmei/jinss/ss3jyomyo.asp