The Bad Romance hitmaker appeared during the Irish rockers' fifth night of their eight-date residency, wearing only black underwear and towering heels as the crowd went wild.

Frontman Bono introduced her to the audience by saying, "Her ladyness, her Gaganess is here this evening."

Gaga played piano and joined Bono for an acoustic duet of the band's track Ordinary Love, which was written to honour late South African activist Nelson Mandela.

U2 shared a picture of the moment on their official page after the show and wrote, "All hail Her Gaganess".

The group previously invited The Roots and U.S. TV host Jimmy Fallon onstage during their residency, which ends on Friday (31Jul15).