PAUL O'CONNELL casts a large shadow. His influence on the teams he plays for – be it Munster, Ireland or the Lions – is always immense.
Filling his sizeable boots for this season will not be an easy task, but Ireland must find a way to minimise his loss. Luckily Donnacha Ryan is developing into a fantastic second- row, while Mike McCarthy showed his many qualities during the November internationals. Unfortunately, neither player is in the flush of youth. Ryan is the younger at 29 years of age and McCarthy is two years older.
When considering the 2015 World Cup, the lack of second-row talent coming through is a huge worry.
It had been hoped Devin Toner would capitalise on his early promise but, in a similar way to Ian Nagle in Munster, he hasn't grasped the opportunities afforded to him at both club and international level.
The last of Toner's three international caps came in 2010. He has been unfortunate with injuries but, even so, has been underwhelming. The same can be said of Nagle, who showed some great early promise. The problem with Nagle is that not enough of him has been seen to know for certain if he is a contender.
It must be hoped one of the academy prospects in the various provinces makes a breakthrough over the next 18 months and challenges the likes of Ryan. It is also likely Iain Henderson will have found a more permanent home in the second-row by then.
That's of little concern to Declan Kidney and Ireland forwards' coach Gert Smal for now, though. They got through the November internationals without O'Connell but navigating the Six Nations will be a different challenge altogether.
Ryan and McCarthy are the incumbents after November. Against South Africa, McCarthy was named man of the match, with Ryan taking the honour a fortnight later against Argentina. Ryan was also one of the few Irish players to emerge from the tour to New Zealand with his reputation enhanced and he continues to impress with every outing in a green shirt.
It is a shame his development was delayed by the presence of so many international-standard second-rows in Munster. Indeed, Ryan has often commented that were it not for his ability to double up as a back-row (blindside), he doesn't believe he would have made the 2011 World Cup squad as a specialist lock.
McCarthy's rise to prominence has been even slower. At 31, it is astonishing that he has only six Ireland caps to his name.
It is even more astounding his international debut came as recently as 2011 given just how well he performed in November, especially as he has been one of Connacht's most consistent performers since he signed from London Wasps in 2003.
But he is determined to make up for lost time now and he will take some shifting for the Six Nations Championship. He will, however, have plenty of competition for that berth, his outstanding shifts against South Africa and Argentina notwithstanding.
So who are the contenders for the second-row for that opening Six Nations game against Wales in Cardiff on February 2?
Ryan is the certainty. He has grown in stature these last few months and is now one of the leading figures in both Munster and Ireland dressing-rooms. He will have responsibility for the line-out, will make the calls out of touch and will be the main target in the No 4 spot.
By virtue of his being the last man to wear the No 5 shirt, McCarthy will certainly be the favourite.
Down in Munster, though, O'Callaghan is in tremendous form and appears reborn under Rob Penney.
His traditional skill of being the man with his nose closest to the grass in rucks is undiminished and he is further showcasing a previously under-appreciated handling ability.
The ease with which he plucked the ball up from his bootlaces from a stray Conor Murray pass midway through the first half of Munster's win over Ulster was phenomenal for such a big man.
His off-loading has also improved hugely, which is another mark in his favour given how Ireland played against Argentina when they moved the ball quickly and sought to create mis-matches behind the scrum. For that tactic to work, the players – forwards as well as backs – must be able to move the ball quickly.
It's probably a safe bet to suggest that the decision is between these two, although an outside bet could be Ulster's Henderson, whose future is in the Ireland second-row even though his two caps have come at blindside flanker.
At six feet six inches and 116kg, he has the look of a second-row. He has good hands, is good in the air, and has the mobility that also suits the high-tempo type of game Ireland played against Argentina and will, presumably, seek to play in the Six Nations.
Ulster's Dan Tuohy and Lewis Stevenson could also come into the reckoning. Tuohy, of course, travelled and played in New Zealand during the summer. He did relatively well in the second Test but, overall, didn't do enough to suggest he could be a medium-term solution. He is also still recovering from a calf injury and would need to figure in Ulster's remaining Heineken Cup pool games to force himself into the equation.
Stevenson is a real outside bet but he has been showing well on occasion for Ulster this season. His knee injury disrupted the end of his 2012 and while he is unlikely to feature in the Six Nations he could be a contender for a seat on the flight to America for the summer tour.