Conor McGregor has become the first fighter in UFC history to hold titles in two divisions simultaneously after a stunning KO of Eddie Alvarez in New York.

The Irish megastar knocked out Alvarez early in the second round at UFC 205 on Sunday (AEDT) to add the lightweight title to the featherweight belt he snared from Jose Aldo in 13 seconds.

McGregor almost put the fight away in the first round after landing a vicious left hook similar to that of the one which destroyed Aldo.

After dominating the first round, McGregor began to showboat before ending the fight.

With Alvarez charging forward, McGregor slipped the punch and replied with a left, right, left and right which sent the New Yorker to the canvas.