2009 Lion Rees hasn't featured for his country since the autumn after a groin injury ruled him out of the opening three rounds of Europe's premier international tournament.
Having won three Lions Test caps on the summer tour of South Africa, Rees has had to sit and watch the defeats to England and France and the tense victory over Scotland but he impressed for the Scarlets in their Magners League win over Ulster on Friday night.
Rees is one of three changes to the Welsh starting line up from the Round 3 clash with the French, with both Ryan and Deiniol Jones unavailable due to injury.
The Welsh captain and 2005 Lion has been ruled out having failed to recover from a calf injury, while the Cardiff Blues second row has withdrawn from the Six Nations squad after damaging his ribs during the first-half of the 26-20 defeat to France.
Triple Lions tourist Martyn Williams will take over the captaincy at Croke Park, with the veteran openside set to lead Wales for the seventh time as he wins his 95th cap.
Gloucester's Gareth Delve replaces Ryan Jones at No8, just days after being called up to the training squad as cover.
"We have taken the decision not to include any players who are unable to take a full part in training in the build up to the Ireland game," said Wales and Lions coach Warren Gatland.
"This Ireland side will be meticulous in its planning and for this game in particular we need to get our preparation exactly right. Once we had reached that conclusion a couple of selection decisions were really made for us.
"Ryan has been managing his calf, but it has hampered his involvement in training and he needs to get it right for his own sake as well as for his region, who have some big matches of their own coming up, and for the sake of the national cause as we countdown to the World Cup.
"But this is an opportunity for Martyn, who is one of our key leaders, to captain Wales again and rise to that challenge after doing a good job against Scotland last year when Ryan was again forced to withdraw."
Newport Gwent Dragons lock Luke Charteris takes over from Deiniol Jones in the boiler house, just as he did during the first half in Cardiff against the unbeaten French.
Rees' promotion to the starting line up means Ospreys hooker Huw Bennett drops to the bench and Ken Owens misses out altogether.
Lions prop Gethin Jenkins is once again not fit for selection, although he did take part in training as he continues his recover from a calf injury.
Sale scrum-half Dwayne Peel, a Test Lion five years ago, is named on the replacements bench instead of 2009 Lion Mike Phillips who was not considered due to a knock he picked up on duty with the Ospreys.
15 Lee Byrne (Ospreys)
14 Leigh Halfpenny (Blues)
13 James Hook (Ospreys)
12 Jamie Roberts (Blues)
11 Shane Williams (Ospreys)
10 Stephen Jones (Scarlets)
9 Richie Rees (Blues)
1 Paul James (Ospreys)
2 Matthew Rees (Scarlets)
3 Adam Jones (Ospreys)
4 Bradley Davies (Blues)
5 Luke Charteris (Dragons)
6 Jonathan Thomas (Ospreys)
7 Martyn Williams (Blues, captain)
8 Gareth Delve (Gloucester)
16 Huw Bennett (Ospreys)
17 Rhys Gill (Saracens)
18 Ian Gough (Ospreys)
19 Sam Warburton (Blues)
20 Dwayne Peel (Sale Sharks)
21 Andrew Bishop (Ospreys)
22 Tom Shanklin (Blues)
Bold denotes Lion