The Mirror World was first accessed in The Flash #126 - Doom of the Mirror-Flash!/Snare Of The Headline Huntress while Samuel Scudder was escaping from prison.
Variations // Changes Throughout History
In the first appearance of the Mirror World it is described as a fourth dimension of Earth that can only be entered through a mirror. The first Mirror Master had been hesitant to venture there due to the fact that he didn't know what awaited him on the other side; however, after needing an escape route when he was in Prison he traveled into the Mirror World for the first time.
In the Silver Age the Mirror World appeared to look like Earth did; however, it's inhabitants were much different, along with the technologies that they possessed. Mirror Master met a tribe of women who called themselves the Orinocas.
The Orinocas resembled Amazonians as they were a group of all women warriors; however, they had never seen a man before nor did they have advanced technology and were impressed with a simple Mirror that Mirror Master had with him.
In the Silver Age Mirror Master was unable to freely exit the Mirror World by his own means and required the assistance of Flash to save him.
Modern Age ( Post Crisis )
The Quickening/Through a Glass Darkly
The first appearances of the Mirror World in the Modern Age is in The Flash #104 - The Quickening and The Flash #105 - Through a Glass Darkly.
In these stories the Mirror World is presented as a sort of Limbo that can only be accessed through reflective surfaces using something the original Mirror Master called "Anti-Tachyons". The second Mirror Master had pulled Flash (Wally) into the realm to try and help him find his ex-lover, Emelia.
In this incarnation of the Mirror World (only seen in these two issues) there are strange beasts made out of shattered Mirrors that re-form when destroyed, duplicating themselves in the process. They are shown to be incredibly hostile as they attack Mirror Master, Flash, and Impulse without any true reasoning behind it.
Also seen in this version of the Mirror World are portals that lead to the real world; meaning it's easier for Mirror Master to transverse the Mirror World as opposed to in the Silver Age.
The Mirror World was revisited during the Wonderland story arc. A powerless Wally West is transported to the Mirror World (along with Captain Cold and Mirror Master (McCulloch)) where everything looks like the city of Keystone; however, the personalities of it's inhabitants are opposite.
Flash is thrown in jail by an alternate version of Heatwave who in this world became a Police Officer for the K.C.P.D.
Two villains known as Plunder and Thinker have learned that the world they are in is an alternate dimension and attempt to use Mirror Master and his equipment to make it back to the real world.
When Mirror Master was able to get Flash and Captain Cold out the experience was described as "The worst headache you've ever had AND spinning out of control".