Conor McGregor just went NUCLEAR at an MMA event in Dublin ... slapping a security guard and pushing veteran UFC ref Marc Goddard for breaking up his post-fight celebration with training partner Charlie Ward.
It all went down after Ward won by 1st-round KO at Bellator 187 -- which prompted McGregor to jump into the cage to show his SBG Ireland teammate some love.
One problem -- Conor wasn't allowed in the cage since he wasn't a licensed cornerman for Charlie ... so Goddard and several security staff members stepped in to interrupt their bro moment.