"I shook my head. "The men in green all forsook England a hundred years ago," said I..." (pg. 124)
The "green men" that Jane is referring to are leprechauns. Leprechauns are tiny little men who were said to live in Ireland and cobble shoes for elves. They have since become a famous fairy tale in their own right, with different versions of their excessive drinking and hidden treasures. Bronte's reference to leprechauns in her novel adds a sense of local color and myth. When Rochester asks Jane if she is in league with the leprechauns, it would be just like if someone in Maine asked if you were spending time with Bigfoot.
EDIT: Bronte's decision to use the leprechaun myth adds to the seemingly mischevious characterization of Jane. Rochester's question of Jane's connection with the leprechauns shows that Rochester finds her impish and even playful. In addition, the reference to leprechauns leaving England points to the longstanding tension between Britain and Ireland.