A male streaker jumped on the stage as Hayden Panettiere read out the nominations for Best Song at Belfast's Odyssey arena.
And the Heroes star feigned surprise, asking the naked man: "Dude, what are you doing?"
Then she invited him to join them for a pint after the show.
Lady Gaga made no mention of the stunt, which was clearly rehearsed, when she collected the gong for Born This Way.
The identity of the man, who spoke with an Irish accent, is unknown.