Peter O'Mahony will become the latest Irish player to reach a landmark 100 caps for his country on Saturday when Ireland face Scotland in a match pivotal to both sides' Rugby World Cup hopes.

"We could sit here all day and talk about what he brings and means to us all, what type of bloke he is, the family man he is," said head coach Andy Farrell of O'Mahony.

"He's selfless. You guys see his performances on the pitch and what it means for him to play for Ireland but behind the scenes, he is definitely, 100 percent, the best I have ever seen at making the dressing room feel right.

"It is him being himself because he's genuine.

"There's no better man you'd want sat alongside you than Peter O'Mahony."

Farrell has brought in hooker Dan Sheehan in preference to Ronan Kelleher whose line-out throwing was awry in the first half of the Springboks match.

Ireland captain Johnny Sexton blamed Kelleher's poor throwing more on bad calls and this may account for Iain Henderson coming into the second row with the line-out caller James Ryan dropping to the bench.

There has been talk of Ryan having a hand injury.

However, his demotion is still a surprise as the Leinster star has been an ever present in the starting XV under Joe Schmidt and then Farrell with him deputising for Sexton as captain when the latter has been unavailable.

"He's fit and he had a bit of a niggle on a wrist but that’s fine," said Farrell.

"He's fit and raring to go."

LIFE AND SOUL

O'Mahony becomes the 10th player to reach the milestone for Ireland, following in the footsteps of current team-mates Sexton, Conor Murray and Keith Earls.

He made his debut for Ireland in a 42-10 win against Italy in February 2012.

Sexton paid a moving tribute to O'Mahony.

"Peter has been a great friend of mine over the last 10, 11, 12 years," said Sexton.

"He's very different to some of the other leaders. You don't get to see the real Peter O'Mahony, he gives very little away in public.

"But he's the life and soul of the group behind closed doors.

"It's a privilege to play with him."

Fellow back row forward Jack Conan is in line to make his first appearance of the tournament as he is on the replacements bench having recovered from a foot injury.

Stuart McCloskey too is primed as the centre gets the nod on the replacements bench due to injury-plagued Robbie Henshaw suffering a hamstring twinge.

"He (Henshaw) pulled up with a hamstring at the end of the session on Tuesday, unfortunate for him," said Farrell.

"The diagnosis with hamstrings, they want to see how things settle down and it probably takes around five days for that kind of thing to happen.

"It looks like it could be a minimum of a couple of weeks at this stage."

Ireland will start as hot favourites to book their ticket for the quarter-finals -- probably against New Zealand -- with a record-extending 17th successive Test victory.

The Scots have it all to do to end a run of eight consecutive defeats at the hands of their opponents despite two morale-boosting wins over Tonga and Romania.

Team (15-1): Hugo Keenan; Mack Hansen, Garry Ringrose, Bundee Aki, James Lowe; Johnny Sexton (capt), Jamison Gibson-Park; Caelan Doris, Josh van der Flier, Peter O'Mahony; Iain Henderson, Tadhg Beirne; Tadhg Furlong, Dan Sheehan, Andrew Porter

Replacements: Ronan Kelleher, Dave Kilcoyne, Finlay Bealham, James Ryan, Jack Conan, Conor Murray, Jack Crowley, Stuart McCloskey