Adam Byrne returns on the right wing in place of Darren Sweetnam with Rory Scholes retaining the full-back berth.
The rest of the back-line - marshalled by captain Luke McGrath and Steve Crosbie - is unchanged as Alex Wootton, a late call-up for the injured Tom Daly, prepares for his second successive start on the left wing.
Darragh Leader and Peter Dooley both miss out through injury with the latter's place at loosehead prop going to Edward Byrne, the twin brother of starting hooker Bryan Byrne.
Gavin Thornbury comes in for Sean McCarthy in the second row and Daniel Leavy has recovered from injury to line out at blindside flanker, the selected back row seeing Conor Joyce revert to the number 8 position.
This is Ireland's second encounter of the tournament with Australia. Ruddock's charges opened their Pool B campaign with a memorable 19-15 win over the Junior Wallabies and they will be looking to repeat that result to claim seventh place overall.
There are nine players in Sunday's team who started the pool opener against the Australians. Wootton is the only personnel change in the back-line - although Scholes was on the wing that day and current centre Rory Scannell was positioned at full-back.
There is a completely new front row in the Byrne brothers and Christopher Taylor, while lock Thornbury and flanker Leavy are the other new inclusions.
IRELAND UNDER-20 Team & Replacements (v Australia Under-20s, 2013 IRB Junior World Championship 7th-8th Place Play-Off, Stade Henri Desgrange, La Roche-sur-Yon, Sunday, June 23, kick-off 3pm local time/2pm Irish time):
15 - Rory Scholes (Belfast Harlequins/Ulster)
14 - Adam Byrne (UCD/Leinster)
13 - Thomas Farrell (Lansdowne/Leinster)
12 - Rory Scannell (Dolphin/Munster)
11 - Alex Wootton (Northampton Saints/Exile)
10 - Steve Crosbie (Old Belvedere/Leinster)
9 - Luke McGrath (UCD/Leinster) (capt)
1 - Edward Byrne (UCD/Leinster)
2 - Bryan Byrne (UCD/Leinster)
3 - Christopher Taylor (Malone/Ulster)
4 - Gavin Thornbury (UCD/Leinster)
5 - John Donnan (Ballynahinch/Ulster)
6 - Daniel Leavy (UCD/Leinster)
7 - Josh van der Flier (UCD/Leinster)
8 - Conor Joyce (Malone/Ulster)
17 - Ryan Furniss (Worcester Warriors/Exile)
18 - Adam Boland (Lansdowne/Connacht)
19 - Sean McCarthy (Shannon/Munster)
20 - Peadar Timmins (UCD/Leinster)
21 - David Shanahan (Clontarf/Leinster)
22 - Mark Roche (Lansdowne/Connacht)
23 - David Panter (Corinthians/Connacht)
IRELAND UNDER-20S' IRB JUNIOR WORLD CHAMPIONSHIP RESULTS/FIXTURE:
POOL B -
Wednesday, June 5: Ireland Under-20s 19 Australia Under-20s 15, Stade de la Rabine, Vannes
Sunday, June 9: Ireland Under-20s 46 Fiji Under-20s 3, Stade de la Rabine, Vannes
Thursday, June 13: New Zealand Under-20s 31 Ireland Under-20s 26, Stade de la Rabine, Vannes
Tuesday, June 18: France Under-20s 9 Ireland Under-20s 8, Stade Henri Desgrange, La Roche-sur-Yon
Sunday, June 23: Ireland Under-20s v Australia Under-20s, Stade Henri Desgrange, La Roche-sur-Yon, 3pm local time/2pm Irish time
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