Cork Constitution fell to their first defeat in four Ulster Bank League games as Alan Kingsley helped Young Munster take a tight 6-3 verdict.
Neither side could manage a try in this muddy affair at Tom Clifford Park, but Munsters surprised even themselves with a hard-earned win against the defending champions and table toppers.
Former Garryowen player Gerry Hurley landed the only score of the first half, a penalty after Young Munster's Willie Staunton was sin-binned for playing the ball on the ground.
Three minutes before the break, the hosts lost influential lock Shane O'Neill to injury and he was replaced by Darragh Payne.
Into the second half, it was Cork Con's turn to have a player sin-binned when Munster prop Stephen Archer incurred the wrath of referee Leo Colgan.
The sides went at it hammer and tongs and scrum half Kingsley drew level before putting the match-winning penalty through the uprights, five minutes from the end.
The scrum half’s second strike sparked great celebrations for the Young Munster faithful, with their side staying in contention for a shot at the Division 1 title.
Next week's trip to current leaders Old Belvedere is a huge game for Mike Prendergast's men, while Cork Con will have to lick their wounds before entertaining Shannon in another heavyweight Munster derby.
Referee: Leo Colgan (IRFU)