James Hook moves from outside centre to out-half, where he started against Scotland last month, as Jonathan Davies returns from a hamstring injury to reform his centre partnership with Jamie Roberts.
Leigh Halfpenny replaces Morgan Stoddart on the right wing, with Stoddart dropping to a replacements bench which also includes Stephen Jones, Dwayne Peel, Richard Hibbard and John Yapp.
There are two more replacement spots to be confirmed after an injury assessment on Thursday. Rob McCusker, Andy Powell and Jonathan Thomas are in contention.
Scrum half Mike Phillips and his Ospreys team-mate Ryan Jones will start against Ireland, winning their 50th caps for Wales.
Head coach Warren Gatland said: "This is a crucial game for us as a win will keep us in the Championship mix and take us into that final round with everything still to play for. But it is also a big game for Ireland who are in a similar position to us going into this match."
Forwards coach Robin McBryde added: "The forwards had a tough day at the office in Italy and we were not at our very best so we are looking for a step up in performance this weekend.
"Ireland will be tough, they have a well-drilled set piece game, are good around the field and are strong defensively.
"They bring intensity to the breakdown area which is always so crucial in these big games and it will be up to us to match them if we are going to be able to provide a platform for our exciting backs to play."
WALES Team & Replacements (v Ireland, 2011 RBS 6 Nations Championship, Millennium Stadium, Saturday, March 12, kick-off 5pm):
15. Lee Byrne (Ospreys)
14. Leigh Halfpenny (Cardiff Blues)
13. Jamie Roberts (Cardiff Blues)
12. Jonathan Davies (Scarlets)
11. Shane Williams (Ospreys)
10. James Hook (Ospreys)
9. Mike Phillips (Ospreys)
1. Paul James (Ospreys)
2. Matthew Rees (Scarlets) (capt)
3. Craig Mitchell (Ospreys)
4. Bradley Davies (Cardiff Blues)
5. Alun Wyn Jones (Ospreys)
6. Dan Lydiate (Newport Gwent Dragons)
7. Sam Warburton (Cardiff Blues)
8. Ryan Jones (Ospreys)
16. Richard Hibbard (Ospreys)
17. John Yapp (Cardiff Blues)
18 and 19. Rob McCusker (Scarlets)/Andy Powell (London Wasps)/Jonathan Thomas (Ospreys)
20. Dwayne Peel (Sale Sharks)
21. Stephen Jones (Scarlets)
22. Morgan Stoddart (Scarlets)