Jimmy Fallon jumped onstage with U2 -- injured finger and all -- and absolutely stole the show ... for at least one song of the Madison Square Garden gig.
Fallon started out by busting on Bono, The Edge, and himself ... for their rash of injuries, saying ... ""I'm a big fan of Irish people who fall down and hurt themselves"
But when the band kicked into their hit, "Desire" -- Jimmy went into lead singer mode ... first sharing the mic, and then taking over and doing a spot-on Bono impression.
You gotta see his harmonica solo at the end. The finger's feeling better.