Replacements: Daniel Montgomery, Gavin Ryan, Mike Madden, John Moroney, Sean Upton.
Replacements: Paddy Hogan, Adam Howard, Ben Marshall, Fergal Lawlor, Peter Robb.
Old Belvedere opened the scoring through their man-of-the-match, impressive young out-half Steve Crosbie, who landed a penalty for the only score of the opening half.
Ireland Under-20 international Crosbie missed two other shots at goal and when winger Derry O'Connor wriggled in to touch down, he had the home side in front after 63 minutes.
But number 8 Jack Conan, another Ireland Under-20 cap in the 'Belvo ranks, grabbed a crucial 70th minute try for the visitors which Crosbie converted.
That looked to have clinched it for the Dubliners but ten minutes from the end the Young Munster pack got up a head of steam and Ger Slattery touched down with a cracking leveller.
It looked set to finish 10-10 until Young Munster conceded yet another penalty and Crosbie slotted over for the full points.
Old Belvedere boss Paul Cunningham, who previously coached Munsters' Limerick rivals Garryowen, praised Crosbie for the composure he showed in slotting that decisive late penalty.
"It was a cracking kick and after missing an easy one in the first half. He is a very young man, a novice in the overall situation but he showed unbelievable character to nail that," Cunningham told the Irish Examiner.
"Overall, I thought it was a very honest performance, the guys rolled up their sleeves and worked very hard. The set piece was strong enough for most of the game. We kicked intelligently and took advantage at the end.
"There was a decent bit of football in the second half and that came from Young Munster too. They played football. They scored a cracking try, the one to go level. They didn't ever stop playing and at the end they were camped on our line. We'll take this one."
Referee: Stuart Gaffikin (IRFU)