The Northampton captain Dylan Hartley describes the Munster crowd as "not so much a 16th man as 17th, 18th and 19th men".
The most eye-catching of the English Premiership sides will therefore find themselves heavily outnumbered as the red-shirted hordes gather this evening for another mighty Heineken Cup occasion at Thomond Park. Can they cope? Hartley believes his men are perfectly capable of beating the former champions and progressing to a third semi-final in eight attempts, but with a game-manager as astute as Ronan O'Gara operating for the opposition, control will be of the essence.
Hence the well-flagged decision to go with Stephen Myler at outside-half, rather than the more flamboyant Shane Geraghty, who is on the bench. It must have been the tightest of calls, and the outcome will leave Geraghty, a first-choice playmaker for England back in the autumn, asking serious questions of himself. But if Myler can kick his goals and run the show sufficiently intelligently to draw the sting from a dangerous Munster midfield, it is not inconceivable that the Midlanders will find a way to win.
Whoever claws their way through will meet the winners of this afternoon's Biarritz-Ospreys tie in the last four. It is not the most daunting prospect in the world: the Welsh regional side are almost wilfully inconsistent, while the Frenchmen are only just rediscovering their equilibrium after a run of seven defeats in 10 domestic championship outings. Will the overtly Basque flavour of big-time rugby in San Sebastian tilt things the way of Biarritz? There is every chance.
In the second-tier Amlin Challenge Cup, tomorrow's Wasps-Gloucester quarter-final guarantees a continuing English presence. The Londoners are beginning to hit their straps and even though Tom Rees, their outstanding open-side flanker, starts this one on the bench, they should have too much firepower for their opponents, despite the return of Mike Tindall in midfield.
Newcastle, meanwhile, will do well to survive tomorrow's set-to with Cardiff Blues, even though they have last season's Heineken Cup semi-finalists on home soil at Kingston Park. Like Ospreys, the Blues find it difficult to string together more than a couple of acceptable performances, but a bristling back-line of international talent must look awfully threatening from where the Tynesiders are sitting.
Indeed, Newcastle will be as interested in what happens in today's two Premiership matches as in their own future in Europe. Worcester, who face London Irish at Sixways, may be adrift by three points, but if both they and Leeds, who visit Harlequins, take something from the weekend, there will be an entire ocean of paranoia sloshing around in the lower reaches come Monday morning.