Mirkwood was the nickname given to a road in Hawkins that went past the Hawkins National Laboratory. Located where the roads Cornwallis and Kerley meet, it was the road that Will Byers would take when going home with his friends.
In November 1983, Will unexpectedly encountered the Monster on Mirkwood while returning home at night. He was forced to run through the forest back to his home, but the Monster chased and subsequently abducted him to an alternate dimension, the Upside Down.
The police found Will's bike by the road the following day, but couldn't find him. Disappointed with the police's efforts, Will's friends, Mike, Dustin & Lucas went to Mirkwood together at night, where they found Eleven in the nearby forest.
At some point before, Dale and Henry went hunting near Mirkwood and disappeared. Bev Mooney, who was presumably close with the two, reported them missing at Hawkins Police Station. Sometime later, in the Upside Down of Mirkwood, when Will was searching for a way out to the real world, he came across Henry who was fatally wounded by the Monster. In his dying moments, Henry requested him to get his friend Dale, who also got injured, back home. As Will offered him solace that they would all return home, the man passed away peacefully. 
Jonathan Byers and Nancy Wheeler went hunting for the Monster on Mirkwood, encountering a wounded deer. As they were about to perform a mercy kill on it, it was suddenly pulled away by an unseen force. Following the trail of blood, Nancy discovered an open portal in the base of a tree. Entering the portal, she found herself in another dimension where she witnessed the creature feeding on the deer. When the Monster was alerted to her presence, she ran away, screaming for Jonathan's help. Following the sound of her voice, Jonathan found the portal and quickly pulled her out to safety. The portal closed immediately after, leaving no evidence that it had ever existed.
Mirkwood is located where the roads Cornwallis and Kerley meet and goes past the Hawkins National Laboratory. It is in close proximity to both the Byers house and the Harrington house, with only a forest separating them.
Behind the scenes
Cornwallis and Kerley is a real intersection in Durham, North Carolina, the hometown of the Duffer Brothers.
The name Mirkwood originates from a forest in Old Norse mythology, where it was called Myrkviðr. It means "dark wood" or "black forest." Inspired by the mythology, Mirkwood is also a forest in The Hobbit and The Lord of the Rings by J.R.R. Tolkien where the character Legolas originated. It used to be called Greenwood the Great before it was taken over by the evil necromancer Sauron.