Sweetnam's academy colleague Rory Scannell replaced Tom Daly after 61 minutes, taking over the kicking duties, and added 1 penalty and a conversion. On his introduction to Ireland U20s rugby, Waterford's Jack O'Donoghue played the full 80.
Ireland play their final pool game against New Zealand on Thursday at 5.45pm Irish time, as both sides go head to head to claim top spot of pool B.
Ireland U20s: Darragh Leader (Galwegians/Connacht); Darren Sweetnam (UCC/Munster), Mark Roche (Lansdowne/Connacht), Tom Daly (Lansdowne/Leinster), Rory Scholes (Belfast Harlequins/Ulster); Steve Crosbie (Old Belvedere/Leinster), David Shanahan (Clontarf/Leinster); Edward Byrne (UCD/Leinster), Bryan Byrne (UCD/Leinster), Christopher Taylor (Malone/Ulster), Gavin Thornbury (UCD/Leinster), Sean McCarthy (Shannon/Munster), Conor Joyce (Malone/Ulster) (capt), Daniel Leavy (UCD/Leinster), Jack O'Donoghue (UL Bohemians/Munster).
Replacements used: Luke McGrath (UCD/Leinster) for Shanahan (53 mins), Josh van der Flier (UCD/Leinster) for Joyce (60), Rory Scannell (Dolphin/Munster) for Daly (61), Ryan Furniss (Worcester Warriors/Exile) for Taylor, John Donnan (Ballynahinch/Ulster) for McCarthy, David Panter (Corinthians/Connacht) for Scholes (all 66), Peter Dooley (Lansdowne/Leinster) for E Byrne, George McGuigan (Newcastle Falcons/Exiles) for B Byrne (both 68), McCarthy for Thornbury (73).
Pool B Fixtures and Results - Junior World Championship 2013 (All times are local)
05 Jun, 18:45 Ireland - Australia Stade de la Rabine, Vannes 19-15
05 Jun, 20:45 New Zealand - Fiji Stade de la Rabine, Vannes 59-6
09 Jun, 18:45 Ireland - Fiji Stade de la Rabine, Vannes 46-3
09 Jun, 20:45 New Zealand - Australia Stade de la Rabine, Vannes 14-10
13 Jun, 18:45 New Zealand - Ireland Stade de la Rabine, Vannes
13 Jun, 20:45 Australia - Fiji Stade de la Rabine, Vannes