"It was the cheapest shot I ever caught in me life, man," Michael McGrane tells us.

Here's the deal ... McGrane took on McGregor in a 4-round fight in Dublin on Friday, April 19. Conor is in the red trunks, Michael is sporting blue.

When officials rang the bell to start the match, McGrane says he expected Conor to touch gloves ... but instead, the UFC superstar unleashed a left hook that caught him flush in the eye.

"You're meant to go back to your corner, but Conor never went back to his corner," McGrane says ... "Conor leaped at me with a left hook."

You can see the damage McGrane says he took from the shot -- still visible days after the fight.

"Conor McGregor hit me a cheap shot in his own hometown!!!"

McGrane says he got no help from the ref that day -- because even the official was obviously rooting for Conor.

"The ref was on his side. There's no way I would have won that fight unless I knocked him out."

By the way, we're told the fight ended in a DRAW.

Despite the controversy, McGrane says he's greatly appreciative of the opportunity to fight someone as HUGE as Conor McGregor and hopes the experience will make him better in the long run.

McGrane -- an electrician by trade -- says he's hoping to give up the electric game to focus on boxing full-time because he thinks he has potential to be a world champ!!!