Conor McGregor put in some violent work Tuesday night -- going "12 hard rounds" with boxing champ Paulie Malignaggi ... and left some marks in the process.
The two got in the ring in Vegas for a full speed workout session to prep for the August 26 superfight with Floyd Mayweather -- and the former WBA welterweight champ tells TMZ Sports, "We both gave each other really good work."
Malignaggi says McGregor been making steady improvements in the ring since the last time they fought 12 days before .