Conor McGregor just went NUCLEAR at an MMA event in Dublin ... slapping a regulatory official and pushing veteran UFC referee 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 fight officials stepped in to interrupt their bro moment.
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McGregor was PISSED -- pushing Goddard from behind and getting in his face ... before the officials and BFF Artem Lobov escorted him from the cage.
But Conor wasn't done. He came back, climbed back on top of the cage and straight slapped one of the officials upside the head.
FYI, Goddard ordered McGregor to leave cageside last time Lobov fought in the UFC ... so that's probably what set Conor off.
Still ... gotta think there's gonna be hell to pay for his actions.