Muro no Yashima (室の八島) was a reference to a Ōmiwa (大神) shrine, which can still be found today in Sōjamachi (惣社町) in Tochigi City to the north of Tokyo. One of the key features of the shrine is a small lake, about eight tenths of an acre in area (2600 square metres) containing eight islets (yashima), each of which has a shrine to a different Shinto deity upon it. Poetically, it was believed that water used to rise from the lake like smoke, and hence it became used as a standard image for smoke which, in turn was a metaphor for the secret fires of love. Unfortunately, the reality of the place does not match up to the poetic image, and a visitor to the shrine will search in vain for water rising from the lake, just as the famous haiku poet Bashō did, two hundred years ago. You can find a picture of the place here.
The name Muro no Yashima literally means “Eight Island Kilns” and occurs in Shinto mythology, where it is said to have been the place where the goddess Konohananosakuyabime (木花咲耶姫命), as she had become pregnant in one night, gave birth to three deities surrounded by fire in order to protect her remaining virtue. It seems likely that word of the place name in distant Shimotsusa reached the capital, and it was then associated with the myth, and hence fire, even though that was not its actual nature.