Munster's forward pressure paid off as they moved into the Irish 22 with O'Mahony stopped just short of the line. The ball was recycled quickly and released out to Cusack who ran in the opening try after 25 minutes, the Garryowen outhalf adding his own conversion.
Within minutes Ireland closed the gap with a James McKinney penalty, before Munster's Craig O'Hanlon scored out wide just before the break to extend the home side's lead to 12-3.
With the Irish coaches ringing the changes for the second half replacement number 10 Paddy Jackson intercepted a Munster ball inside his own 22 to run the length of the pitch and score the visitors first try. Jackson missed the conversion but followed up with a penalty after 64 minutes to make it 12-11.
Munster continued to press on and a decision to kick for touch rather than go for points proved worthwhile when Nagle powered over the line after 70 minutes, but everything changed in the closing minutes as Munster conceded two late tries.
The young Irish side showed great resilience and composure as they continued to attack, running the ball and spreading it wide with wingers Andrew Boyle and Mick McGrath both scoring before the final whistle.
Result: Munster Development 17 - Ireland U20s 21
Munster Development: L O'Dea (Shannon), C O'Hanlon (Young Munster), C Bohane (Dolphin), L Kuntz (Garryowen), C Quinn (Cork Constitution), D Cusack (Garryowen), B Slater (UCC), J Cronin (Highfield), N Scannell (Dolphin), J Ryan (UCC), I Nagle (Cork Constitution), B Hayes (Cork Constitution), P O'Mahony (Cork Constitution), B O'Hara (UL Bohemians), P Butler (Shannon). Replacements: J Holland for Kuntz (32, injured), M Dolan (Irl U20) for Slater (35), D Casey (Shannon) for Scannell (37), Hircock for Quinn (48), P O'Regan (UL Bohemian) for Ryan (50), J Quill (Dolphin) for O'Hara (54).
Ireland U20s (First half): T O'Halloran (Galwegians/Connacht), A Boyle (UCD/Leinster), C Hircock (Garryowen/Munster), JJ Hanrahan (UL Bohemian/Munster), C O'Shea (Clontarf/Leinster), J McKinney (Queens/Ulster), K Marmion (UWIC/Exiles), F Belham (Belfast Harlequins/Ulster), D Doyle (UCC/Leinster), T Furlong (Clontarf/Leinster), D Qualter (Buccaneers/Connacht), C O'Flaherty (Cork Con/Munster), S Buckley (Garryowen/Munster), J Murphy (Lansdowne/Leinster), A Conneely (Corinthians/Connacht). Replacements: L Marshall (Ballymena/Ulster) for O'Halloran, N Annett (Belfast Harlequins/Ulster) for Doyle (both28), C Carey (Ballymena/Ulster) for Furlong (38)
Ireland U20s (Second half): L Marshall (Ballymena/Ulster), M McGrath (Clontarf/Leinster), B Macken (Blackrock/Leinster), N McDonald (Exiles), A Boyle (UCD/Leinster), P Jackson (Dungannon/Ulster), P Du Toit (UCD/Leinster), A Warwick (Ballymena/Ulster), N Annett (Belfast Harlequins/Ulster), C Carey (Ballymena/Ulster), M Kearney (Clontarf/Leinster), I Henderson (Queens/Ulster), S Buckley (Garryowen/Munster), J Murphy (Lansdowne/Leinster), C O'Flaherty (Cork Constitution/Munster). Replacements: D Doyle (UCD/Leinster) for Henderson (48)
Referee: J Carvill (IRFU)