Fresh from scoring his first senior try for Ulster, Marshall will reform his centre partnership with Alex Kelly and Gilroy, Ulster's top try scorer in the league, returns on the right wing.
They are the only two changes to the team that defeated Scotland 15-0 last time out. JJ Hanrahan and Tiernan O'Halloran make way and take up spots on the replacements bench.
Looking forward to building on that win in Inverness, Ruddock said: "We saw a big improvement, particularly in terms of our line speed in defence and in the collision which we dominated a lot more than we did against France.
"That gives us some confidence going to Wales. It will be extremely tough against the Welsh side as they seem to be playing with great confidence.
"We need to ensure our defence is spot on and take our chances when they come along."
Tickets for Friday's match are priced at £10 for adults and £5 for Under-16s and are available now from the Parc y Scarlets ticket office (tel: 08718718088) and from Ticketmaster (tel: 08442777888). Click here for directions to the ground.
The game will also be broadcast live on RTÉ Two (coverage begins at 7pm), with commentary provided by Ryle Nugent and Ralph Keyes.
IRELAND UNDER-20 Team & Replacements (v Wales Under-20s, 2011 RBS Under-20 6 Nations Championship, Parc y Scarlets, Llanelli, Friday, March 11, kick-off 7.10pm):
15 - Michael Sherlock (Clontarf/Leinster)
14 - Craig Gilroy (Dungannon/Ulster)
13 - Alex Kelly (UCD/Leinster)
12 - Luke Marshall (Ballymena/Ulster)
10 - Paddy Jackson (Dungannon/Ulster)
9 - Blane McIlroy (Ballymena/Ulster)
1 - James Tracy (UCD/Leinster)
2 - Niall Annett (Belfast Harlequins/Ulster) (capt)
3 - Martin Moore (Lansdowne/Leinster)
4 - Michael Kearney (Clontarf/Leinster)
5 - Daniel Qualter (Buccaneers/Connacht)
6 - Shane Buckley (Garryowen/Munster)
7 - Aaron Conneely (Corinthians/Connacht)
8 - Jordi Murphy (Lansdowne/Leinster)
16 - David Doyle (UCD/Leinster)
17 - Tadhg Furlong (Clontarf/Leinster)
18 - Iain Henderson (Queen's University/Ulster)
19 - Cathal O'Flaherty (Cork Constitution/Munster)
20 - Mark Dolan (Corinthians/Connacht)
21 - JJ Hanrahan (UL Bohemians/Munster)
22 - Tiernan O'Halloran (Galwegians/Connacht)