Ireland face arguably their biggest test of a potential title-winning RBS 6 Nations season when they tackle France in Dublin on Saturday.
Eddie O'Sullivan's team are racing towards a first Grand Slam since 1948, having already seen off Italy, Scotland and England.
Wales will await them at the Millennium Stadium on Saturday week, when both sides could be chasing the title and championship clean sweep.
The problem for O'Sullivan and his players though, is that he doesn't quite know what to expect from France.
The French, awful against Scotland last month, but then brilliant for large parts of a thrilling struggle with Wales that they eventually lost, cannot secure a second successive Grand Slam, although Fabien Pelous and company can still retain the Six Nations crown.
France, there is no doubt, have the ability to beat Ireland this weekend. The big issue is a psychological one, and whether or not the mental state will be right for what could be a cracking Test match.
O'Sullivan, as the dust settled on Ireland's victory over England a fortnight ago, openly admitted that it is virtually impossible to gauge the French mood.
If France "turn up" though, then Irish hopes of Six Nations success will receive a severe examination.
O'Sullivan has made one enforced change from the side that accounted for England, calling up experienced Ulster centre Kevin Maggs instead of an injured Shane Horgan.
Maggs will be skipper Brian O'Driscoll's third different midfield partner in this season's tournament.
France, with skipper Pelous winning his 100th cap to join fellow international centurions Jason Leonard, Philippe Sella, George Gregan and David Campese, lost narrowly in Dublin two years ago.
They will be out to avenge that defeat, and no-one should be surprised if the Irish bubble is burst, but home advantage might just be enough to swing the pendulum Ireland's way.
G Murphy (Leicester); G Dempsey (Leinster), B O'Driscoll (Leinster, capt), K Maggs (Ulster), D Hickie (Leinster); R O'Gara (Munster), P Stringer (Munster); R Corrigan (Leinster), S Byrne (Leinster), J Hayes (Munster), M O'Kelly (Leinster), P O'Connell (Munster), S Easterby (Llanelli), J O'Connor (Wasps), A Foley (Munster).
Replacements: F Sheahan (Munster), M Horan (Munster), D O'Callaghan (Munster), E Miller (Leinster), G Easterby (Leinster), D Humphreys (Ulster), G Duffy (Harlequins).
England Under-20s head coach Martin Haag was full of praise for his World Rugby Under-20 Championship-winning side after Harry Mallinder inspired them to a 45-21 win over their Irish counterparts and a third title in four years.