The countdown to Ireland's Six Nations title defence has truly begun after head coach Andy Farrell unveiled the squad he hopes can deliver back-to-back Grand Slams.

With injuries and retirements, there are eight players in the 34-man squad who were not part of Farrell's group for the World Cup.

Here, BBC Sport NI looks at five talking points from Ireland's Six Nations squad.

O'Mahony beats stiff competition for captaincy

For a while, O'Mahony's Ireland future appeared uncertain. In the wake of another World Cup quarter-final, he said he was undecided about whether or not he would continue his Test career.

That uncertainty was deepened when the 34-year-old flanker surprisingly stepped down as Munster captain in November after a decade leading the province.

And while O'Mahony's Irish Rugby Football Union contract is set to expire this summer, the fact that Farrell has rewarded the back row for his loyalty over the years could be a sign that he will soon agree fresh terms.

O'Mahony fended off stiff competition for the captaincy. Some felt James Ryan, who was vice-captain under Sexton, was the obvious choice, while Caelan Doris, Iain Henderson and Garry Ringrose had all been mentioned as candidates.

And while O'Mahony is approaching the winter of his career, he has been one of Ireland's beating hearts over the past decade.

A hugely passionate player who once captained the British and Irish Lions, he commands the respect of the rest of the squad and will throw everything into the role.

Crowley gets his big chance

The captaincy was one of the issues Farrell needed to address following Sexton's retirement. The other was the void he left in the team.

For so long, Sexton loomed large over Ireland's fly-half hopefuls, and while Joey Carbery and Billy Burns have seen opportunities to stake their claim come and go, Jack Crowley has emerged as the man tasked with maintaining the standards set by his predecessor.

Having just turned 24, Crowley has much to learn, but having already showcased his playmaking qualities during Munster's run to the United Rugby Championship title last season, there is clearly a belief among the coaching staff that he has the skillset to become a hardened Test player.

It is, however, a big step up. Crowley's biggest start in green to date was an autumn win over Australia in 2022. Assuming he is named at 10 for Ireland's Six Nations opener in France, he will be exposed to a whole new level of pressure and responsibility.

Keeping Crowley fit will also be key. Were he to succumb to injury, Farrell's other fly-halves in the squad - Harry Byrne and Ciaran Frawley - have just three caps between them.

Ulster players rewarded for form

There were four Ulster players in Farrell's 33-man World Cup squad, and while Rob Herring misses out because of injury, there are six representatives from the northern province in the Six Nations panel.

They are there on merit, too. Ulster skipper Henderson is well established in the second row at this stage, while Stuart McCloskey did enough at the World Cup to remain in the midfield mix.

Tom O'Toole is a prop with a bright future while prolific try-scoring hooker Tom Stewart, who made his Test debut in the World Cup warm-up game against Samoa, has been given another chance to prove his mettle following Herring's injury.

Jacob Stockdale and Nick Timoney both played their way back into Farrell's graces, too. Stockdale, the golden boy of Ireland's 2018 Grand slam success, has been in excellent form for his club after missing out on World Cup selection.

Timoney, who won the last of his three caps in 2022, has clearly caught Farrell's eye after scoring two tries in Ulster's nail-biting New Year's Day win away to Leinster.

A versatile back-rower whose try-scoring capabilities are supplemented by his proficiencies as a tackler, Timoney may prove useful to Farrell over the next two months.

Who replaces Hansen on the wing?

There were some notable absentees from Farrell's squad, with Connacht's Mack Hansen chief among them.

The Canberra-born wing, who has become immensely popular with Irish fans since exploding on to the scene at the 2022 Six Nations, will miss the tournament with a shoulder injury.

James Lowe is a nailed-on starter on the left wing, but it is less clear who will take Hansen's spot on the right.

With the versatile Jimmy O'Brien unable to step in because of injury and given that Stockdale is predominantly a left winger, it may be Jordan Larmour's chance to make up for lost time.

Larmour, like Stockdale, emerged as a hugely exciting proposition in 2018, earning a nomination for the World Rugby breakthrough player of the year award.

The 26-year-old has endured his fair share of injury troubles, earning the last of his 30 caps in 2021, but on Saturday offered a thrilling reminder of his destructive abilities by scoring two tries in Leinster's thrashing of Stade Francais - a match Farrell watched from the stands.

At the other end of the experience spectrum stands Calvin Nash. He is the same age as Larmour, but has just one cap (against Italy last year). He is, however, a hugely exciting player with ball in hand.

It's a big call for the Ireland coach.

Those who missed out

Like with any squad announcement, there are those who will be disappointed not to have made the cut.

While uncapped fly-half Sam Prendergast is included as a training panellist, his brother Cian must wait to add to his three caps having been edged out of the back-row picture by Timoney.

Gavin Coombes, a number eight with two caps, falls into the same category while club-mate Simon Zebo's fine form for Munster failed to yield a recall, having last graced the Test stage in 2017.

Ulster's Rob Baloucoune and James Hume may also feel disappointed, but if Farrell runs into injury problems with his wings or centres, they may expect a call.