It's a job he always wanted and with 10 consecutive wins to his credit this year, one could expect him to be extremely happy with his lot. Now, however, he must face up to the most difficult task of all settling on the team he believes is best equipped to complete the Grand Slam against England at Lansdowne Road on Sunday.
He is fortunate that Saturday's thriller against Wales left him with no further injury problems and that those who missed out on Cardiff, such as Gary Longwell and Victor Costello, should be available for selection. But it is who to exclude, rather than who to select, that leaves him with the greatest level of soul searching.
By my reckoning, there are a minimum of three places in the side and possibly as many as five where he has major decisions to make.
The pubs, coffee houses, street corners, and just about anywhere you can think of, have been full of the David Humphreys/Ronan O'Gara outhalf scenario since the Cork Constitution man proved his fitness in time to be considered for the game against France. It has become more animated since O'Gara was called off the bench in Cardiff and dropped the goal that saw off the Welsh by a single point.
There is always a danger of provincialism entering this kind of scenario. Hopefully, that can be left aside on this occasion for never in the history of Irish rugby has there been a game of this importance.
Whichever way O'Sullivan leans, there will be those who believe he got it wrong. Ulster says "no" where dropping Humphreys is concerned, but Munster men are equally vocal in espousing O'Gara's virtues.
O'Sullivan's decision will, I suspect, be largely governed by the type of team he intends putting on the field.
Will he opt for the tried and trusted of the past few weeks which would mean Humphreys at outhalf and Gary Longwell in the second row? Or will he take the more adventurous route of going for O'Gara at No 10 and the likes of Paul O'Connell or Donncha O'Callaghan as Malcolm O'Kelly's second row partner?
For all his virtues, you have to doubt that Leo Cullen commands the physical presence to cope with a challenge of this magnitude, both around the field and in the set pieces.
The indications are O'Sullivan will settle for Humphreys and Longwell and that would be especially hard luck on O'Gara, who owned the outhalf position until Brett Sinkinson intervened in the Celtic League final. Longwell is the solid, reliable type and probably deserves a recall, but I would prefer the physicality of O'Connell or O'Callaghan.
True, O'Connell is short of match practice but even his introduction as an impact player for the final 20 minutes or so could have hugely beneficial results for Ireland.
The third big question mark hangs over the right wing berth. If Shane Horgan and Girvan Dempsey were both available it would probably be a contest between Horgan and Geordan Murphy.
Now, though, Murphy will definitely remain at fullback with Justin Bishop and John Kelly battling it out for the No 14 jersey. It is maintained a missed tackle by Kelly in the French game cost him his place, but even if that is the case, it was an aberration.
A great man to take his try scoring opportunities (remember Munster v Gloucester, not to mention his two tries against Italy), he is also hugely reliable in all facets of the game.
Bishop got the nod for Cardiff and as the man in possession may well hold on. If so, that would be extremely hard on Kelly and, of course, represents another tough call for O'Sullivan.
A case could also be made for the inclusion from the start of Justin Fitzpatrick ahead of Marcus Horan and for Alan Quinlan to hold on at number six before Victor Costello.
The Irish scrum creaked in Cardiff although referee Steve Lander didn't help in this respect, seeming to be as bewildered as the next man as to exactly what was happening in there.
Fitzpatrick is more physically commanding than Horan (6' 2" and 18 stone against 6'1" and 16st 7) and is a lot more experienced.
However, Horan proved himself against Phil Vickery (who, crucially, will be an English absentee) against Gloucester in the Heineken Cup and against Sylvain Marconnet and Christian Califano in the French game. He will almost certainly start, as will Costello, recovered from the knock that kept him out in Cardiff where Quinlan had another splendid outing, only to undo a lot of the good by conceding a penalty in a kickable position at a crucial stage.
That he was taken off shortly afterwards suggests the incident could well be his undoing for Sunday.
IRELAND XV (probable): Murphy; Bishop, O'Driscoll, Maggs, Hickie; Humphreys, Stringer; Horan, Byrne, Hayes; Longwell, O'Kelly; Costello, Gleeson, Foley.
Reserves: Sheahan, Fitzpatrick, Cullen, Quinlan; G Easterby, O'Gara, Kelly.