Try scorer Rory Scholes, goal-kicker Tom Daly and out-half Steve Crosbie are the three survivors in the back-line, with blindside flanker Conor Joyce retaining his place and captaining the side in the absence of Luke McGrath.
Connacht's Darragh Leader has been named at full-back with ex-Cork hurler Darren Sweetnam and Scholes completing the back-three.
Mark Roche joins his Lansdowne club-mate Daly in the centre, while Clontarf scrum half David Shanahan will make his full debut at Under-20 level.
Twins Edward and Bryan Byrne are also set for their first starts of the campaign in an all-new front row alongside Christopher Taylor. Gavin Thornbury and Sean McCarthy, a nephew of former Ireland captain Mick Galwey, make up the second row pairing.
Daniel Leavy, an impressive replacement against the Australians, and Jack O'Donoghue - the last of the five players not involved in the Six Nations to see Junior World Championship action - link up with Joyce in the back row.
Speaking ahead of the clash, Ruddock said: "We want to build on a big win against Australia with a top performance against Fiji.
"We have freshened the side up quite a bit, the heat and physicality of the Australian game combined with a four-day turnaround means we have to use all our resources to raise our performance."
Sunday evening's game between Ireland and Fiji will be broadcast live on TG4 and Sky Sports 4. The Fijians are hoping to bounce back from a 59-6 loss to New Zealand in their tournament opener.
IRELAND UNDER-20 Team & Replacements (v Fiji Under-20s, 2013 IRB Junior World Championship Pool B, Stade de la Rabine, Vannes, Sunday, June 9, kick-off 6.45pm local time/5.45pm Irish time):
15 - Darragh Leader (Galwegians/Connacht)
14 - Darren Sweetnam (UCC/Munster)
13 - Mark Roche (Lansdowne/Connacht)
12 - Tom Daly (Lansdowne/Leinster)
11 - Rory Scholes (Belfast Harlequins/Ulster)
10 - Steve Crosbie (Old Belvedere/Leinster)
9 - David Shanahan (Clontarf/Leinster)
1 - Edward Byrne (UCD/Leinster)
2 - Bryan Byrne (UCD/Leinster)
3 - Christopher Taylor (Malone/Ulster)
4 - Gavin Thornbury (UCD/Leinster)
5 - Sean McCarthy (Shannon/Munster)
6 - Conor Joyce (Malone/Ulster) (capt)
7 - Daniel Leavy (UCD/Leinster)
8 - Jack O'Donoghue (UL Bohemians/Munster)
16 - George McGuigan (Newcastle Falcons/Exile)
17 - Peter Dooley (Lansdowne/Leinster)
18 - Ryan Furniss (Worcester Warriors/Exile)
19 - John Donnan (Ballynahinch/Ulster)
20 - Josh van der Flier (UCD/Leinster)
21 - Luke McGrath (UCD/Leinster)
22 - Rory Scannell (Dolphin/Munster)
23 - David Panter (Corinthians/Connacht)
IRELAND UNDER-20S' IRB JUNIOR WORLD CHAMPIONSHIP FIXTURES/RESULTS:
POOL B -
Wednesday, June 5: Ireland Under-20s 19 Australia Under-20s 15, Stade de la Rabine, Vannes
Sunday, June 9: Ireland Under-20s v Fiji Under-20s, Stade de la Rabine, Vannes, 6.45pm local time/5.45pm Irish time (live TG4/Sky Sports 4/HD4)
Thursday, June 13: New Zealand Under-20s v Ireland Under-20s, Stade de la Rabine, Vannes, 6.45pm local time/5.45pm Irish time (live TG4/Sky Sports 3 red button/Sky Sports Xtra)
Tuesday, June 18 and Sunday, June 23
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