Young Munster shocked Cork Constitution into submission at Temple Hill as the defending champions suffered their first AIB League defeat since last January.
Constitution's 11-match winning run came to an end as Young Munster full-back Matt Costello and centre Glen Carroll claimed second half tries.
Coming off the back of last week's gritty 9-7 success at UCD, the Leesiders were determined to make a positive start and they got one when flanker Billy Holland notched a fifth-minute try.
They sent Munsters on the retreat, controlling the ball and building through the phases before Holland found enough space to score out wide.
But by the 24th minute, the visitors were 9-5 ahead. Costello bisected the posts on the quarter-hour mark with a well-judged penalty from the 22.
He repeated the dose two minutes later and was then on target from 40 metres for a hat-trick of penalty successes.
However, Con snapped back the lead before half-time - 10-9 - when winger Richie Lane beat two defenders on a powerful run from the right.
Lane's fourth try of the league season looked to have settled the Con side going into the break.
Yet Munsters dug deep and stunned the champions when Costello burst through for a 46th-minute try.
Costello brilliantly intercepted a pass in the 22 and darted forward to score under the posts. He added the conversion himself for a 16-10 Cookies lead.
Con, with captain Brendan Cuttriss keeping his side patient, won a penalty in the 63rd minute, which Lane sent through the posts.
However, Young Munster replied with a fine effort from Carroll to stretch their lead to eight points and in the end, with Lane kicking a 70th-minute penalty, Con had to settle for a losing bonus point.
Scorers:
Cork Constitution: Tries: Billy Holland, Richie Lane; Pens: Richie Lane 2
Young Munster: Tries: Matt Costello, Glen Carroll; Con: Matt Costello; Pens: Matt Costello 3
