Conor McGregor DANCED HIS FACE OFF at Sister's Wedding (VIDEO)

Conor McGregor's always ready to fight ... even when he's dancing.

The UFC superstar let the beat take hold at his sister Aoife's New Year's Eve wedding in Ireland -- and unleashed some dance moves that would even make Michael Flatley jealous.

Dude gets so hyped, he starts throwing punches at one point ... while standing right next to Aoife's husband, Mark Elliott.

Lucky for everyone, he didn't connect.