Of Joe Schmidt's preferred pack at the World Cup, Mike Ross, Paul O'Connell, Ian Henderson and Peter O'Mahoney will likely miss the 6 Nations. Given the pack was Ireland's area of strength in the World Cup, that's a serious blow. It also perhaps means that a switch to a more open game is now a necessity since Ireland won't have the depth to bully teams up front.
As such, the make up of the midfield is the most intriguing aspect of Joe Schmidt's selection. Luke Fitzgerald, Stuart McCloskey, Luke Marshall, Robbie Henshaw and Garry Ringrose are all putting their hands up for selection. All are likely to be around for the 2019 World Cup too with Stuart Olding also giving Ireland a tremendous set of options. I think Joe Schmidt will opt for a midfield of 12 Fitzgerald and 13 Henshaw with Luke Marshall on the bench for the start of the 6 Nations. McCloskey and Ringrose will likely have to bide their time and get used to the systems employed but both have huge careers for Ireland ahead of them.
Going on a tangent, Clive Woodward's concept of gold, silver and bronze medal players has some merit - by the time the World Cup comes around, everybody in the starting XV should be among the top three players in his position in the world. The three years before a World Cup are the time for experimentation with the final year all about consolidation.
Following Woodward's concept, it's easy to be bold about selections when you don't have to justify a poor performance after said selections fail! As such, I'd like to see Ireland be brave in their team selections in the 6 Nations with an eye on 2019 whereas I don't believe Joe Schmidt will be as radical in his overhaul. A failure of the 2015 World Cup was lack of depth (which David Nucifora is being tasked with addressing) and an internationally inexperienced centre competition. Robbie Henshaw and Jared Payne didn't have enough time to gel and Ireland had no real alternatives. Brian O'Driscoll was allowed hang on too long and it hurt Ireland in my opinion. Now is the time to put the ills of 2015 right. I'd go with the following as Ireland's optimum selection based on who is fit:
15. Simon Zebo (For his game breaking threat rather than his form. Rob Kearney provides nowhere near enough counter attacking threat and will be 33 in 2019)
14. Keith Earls
13. Luke Marshall
12. Robbie Henshaw
11. Luke Fitzgerald
10. Jonny Sexton
9. Conor Murray
1. Jack McGrath
2. Rory Best (captain)
3. Marty Moore (Tadhg Furlong splits time almost equally, Nathan White and Mike Ross won't make it to 2019 and should be left out even though they're better right now)
4. Devin Toner
5. Mike McCarthy (Ultan Dillane, Ross Molony and Donnacha Ryan should be in the squad. Ryan as a potential starter, the others for their future potential)
6. Rhys Ruddock
7. Tommy O'Donnell (ball carriers needed in a pack shorn of Healy and Henderson)
8. Sean O'Brien (Ireland need to develop alternatives to the 32 year old Jamie Heaslip)
What I expect:
Kearney, Bowe, Henshaw, Fitzgerald, Trimble, Sexton, Murray, McGrath, Best, White, Toner, McCarthy, O'Brien, Henry, Heaslip