The mirror-universe Martok is a coarse, bullying Klingon officer aboard Regent Worf's flagship, seen in The Emperor's New Cloak, with none of the dignity of the general known on Deep Space 9.
The mirror counterpart of Martok appears once, in the seventh-season episode The Emperor's New Cloak, played by J.G. Hertzler, who also plays the prime-universe general.
Where the Martok of the prime universe is a thoughtful soldier who has earned his rank the hard way, this one is a crude and boastful subordinate of Regent Worf in the Klingon-Cardassian Alliance. He serves aboard the Regent's flagship while it waits for Quark and Rom to deliver a cloaking device.
He is aboard when the sabotaged cloak cripples the ship in battle with the rebels' Defiant and the Regent has to surrender. The contrast with his counterpart is the main point of the character.