Conor McGregor is FINALLY opening up about why he threw a metal dolly through a bus window ... albeit he ain't the real McGregor ... he's an impersonator.

The guy's name is Al Foran -- and his McGregor impersonation is spot on ... complete with expensive sunglasses and a thick accent ... and he's giving TMZ Sports the perfect take on why the Irish fighter went off in Brooklyn.

"I was just trying to give the dolly over to the Russian," fake Conor says.

"To whatever his name is -- Kabob. I wanted to hand it to him and unfortunately it went through the bus window!!"

That ain't all ... fake Conor shows us how the UFC superstar might be spending his five days of community service -- and also takes an epic shot at a TMZ Sports photog that got Conor into deep water with the religious folks a few years ago.

BTW ... Al's done this Conor bit before -- and even got a few chuckles out of Floyd Mayweather from it!