The biggest is what influence Nonu will have on Kirwan's younger players, especially Francis Saili, a new All Black who happens to play in Nonu's preferred second-five position.
Nonu has earned a reputation, rightly or wrongly, of being a divisive figure - a bad influence, something which his migration around three teams in three seasons did nothing to dispel. Now he's back at the Blues, effectively because no other New Zealand franchise wanted him.
There are rumours that Nonu was a big factor in Aaron Cruden leaving the Hurricanes for the Chiefs - any similar pressure on other players within the Kirwan's Blues squad could have a catastrophic effect.
Having done such good work in building what appeared to be a close and harmonious squad in his first year in charge - and the on-field results were extremely positive before dropping off at the end of the season - any difficulties posed by a player with Nonu's track record will unravel that good work and more.
It's not Nonu's fault, but his arrival is understood to have already effectively ended the Blues career of Malakai Fekitoa, a 100kg midfielder who played every match of Auckland's 2012 ITM Cup campaign but who didn't make an appearance for the Blues this year despite being seen as one for the future.
With Rene Ranger moving to France, Fekitoa must have been excited about his opportunity to push his case for next season, but Nonu's signing has squashed that. It is understood that he has signed with the Highlanders.
These are all things Kirwan must have weighed up, along with Blues assistants Graham Henry and Mike Byrne, a former All Blacks head coach and current assistant, respectively.
The upsides are compelling. Nonu has played 85 tests and 133 Super Rugby matches and is one of the best midfielders in the world. He is such a consistent player for the national team that his performances in Super Rugby - especially for the Highlanders - are not so much below par as beyond belief, yellow cards and all.
He joined the Blues under Pat Lam at the start of last year following a stint for Ricoh Black Rams which meant he had no down time between the World Cup victory and the Blues' wretched 2012 season.
The most memorable moments of his season at the Highlanders this year was his yellow card for a late shoulder charge on mate Piri Weepu on his return to Eden Park, and a red card for a tip tackle on Crusader Tom Marshall in Dunedin. The Highlanders won three games and lost 13.
Now he is back. There isn't much pressure on him to perform - he's in the All Blacks regardless - but there's plenty on Kirwan to get the best out of him.