Vincent Hogan - Cardiff encounter set to become football as a fight
Part of tonight's fascination is in imagining Ireland's dressing-room at the moment of pre-match exit, in trying to reconcile the bespectacled, academic reserve of Martin O'Neill with the machine-gunner's eyes of his assistant.
How exactly are the two compatible?
From the outside, their union is hard to figure, a literary collaboration between Jack Kerouac and Lewis Carroll. Four years into this gig, we still struggle to understand the chemistry, to work out how two personalities of such contrasting heat find common philosophical ground on these seismic evenings.
After the Moldova game on Friday, O'Neill was invited to consider the possibility that, given Seamus Coleman's horrible leg-break last March, an element of "needle" might contaminate tonight's proceedings. He seemed puzzled, even mildly affronted by the suggestion. It simply wasn't a narrative he had any desire to explore.
Asked about a need to take early heat out of this game with Wales, he responded with bemusement.
"But I don't know if there'll be any heat in it!"
It was a sensible response, yet one you had to suspect didn't quite come from the heart. For if it remains difficult to prophesise much about tonight in Cardiff's Leckwith district, we do know the game won't be an essay in grace or elegance. With both sides compelled to chase victory in a raucous derby atmosphere, success will have to be - above all - a triumph of nerve, of checks and balances.
And, from this untidy jigsaw, there is the distinct possibility of both sides falling short.
For O'Neill especially, fresh from that faintly ecumenical FAI TV video message confirming his decision to continue as Irish manager, this could be uncomfortable. A bold, authoritative start to the qualifying campaign has receded into an anxious trudge and, after tonight, Ireland's itinerary for the next ten months could well be a tepid procession of friendlies.
At the very least then, he needs a performance that communicates some kind of growth and tactical clarity.
Wales believe, palpably, that - even without Gareth Bale - they are better than Ireland and fully expect a partisan home crowd to accentuate that advantage. Joe Ledley, a journeyman midfielder of theirs released last season by Crystal Palace and, subsequently, a late addition to the books of Leeds, reads it thus: "Monday will be bouncing and we'll make sure that we're on it when we cross the line."
Yet, the last time Wales beat Ireland in football, Jack Charlton was a fledgling international manager, still some distance away from winning the Irish public over.
O'Neill's view is that tonight's game was, from the outset of Group D, pre-destined to be decisive, yet the truth is there was little indication it would carry that status when Ireland topped the table after victory in Vienna last November. What's happened since has felt like some kind of collective loss of nerve.
So O'Neill and Roy Keane need to find the best of themselves here, and of each other.
They both speak reverentially of the late Brian Clough, the ultimate maverick leader who - famously - encouraged his players to have wine with lunch on big match-days to help them sleep in the afternoon and once bought a dozen stools so that they could sit with their feet up in the dressing-room.
Yet recent evidence suggests this management team might be cutting its cloth closer to the obdurate Charlton constitution, an irony given Keane's insistence in his autobiography that "Charlton merely blustered, was short on detail, long on generalisations to do with 'putting them under pressure' and 'getting tight' on your man."
On Friday night in Lansdowne, both Irish goals came from the kind of direct football promoted by Charlton and there was little if any evidence throughout of defenders or midfielders doing anything but turn their backs whenever goalkeeper, Darren Randolph, had possession.
Whether that was down to managerial dictate or players lacking the confidence to take responsibility is open to conjecture. What is beyond argument, however, is that four years into this relationship, any sense of Ireland playing modern, progressive, ambitious football has largely evaporated.
O'Neill was keen on Friday to highlight management's appetite for a gamble in that time, citing the selection of David Meyler at full-back against Germany in Gelsenkirchen and Shane Duffy's elevation to a starting role against Italy in Lille as examples of bold management. "In big, big matches, I think we've taken a lot of risks with players" he said.
"I think if you look over our time here, we have done a lot of risky things and we may have to again."
It's certainly true that his team selections have been all but impossible to predict, but to promote a suggestion that those selections represent some kind of nerveless, innovative leadership would be subscribing to fiction.
International management is largely about plugging holes and building spirit. Charlton did that brilliantly, albeit with far better players to work with than O'Neill has the blessing of today. That said, it was interesting to read extracts from Graeme Souness's autobiography in a Sunday newspaper yesterday lauding Charlton as the manager under whom he "turned from a boy into a man."
There wasn't much intricacy to how Charlton worked, given his belief that physical dominance was the pragmatist's way of winning football games.
A lot of people did not like that, but it was at least clear-cut, unapologetic, consistent. Mick McCarthy favoured greater trust in players, more passing; Brian Kerr didn't get the time to build a discernible style; Giovanni Trapattoni preferred a starkly regimented version of football by numbers.
It's true, there have been glimpses of a better way under O'Neill, but tonight's circumstances highlight how that vision has begun to blur.
Bottom line, the team needs to be aggressive now whilst tuning out of all the angry turbulence around them. The game will, almost certainly, pass in a rush. It won't wait for men to find their breath and, if we're honest, this promises to be one of those nights where philosophies lose their anchorage pretty quickly. In other words, it looks set to become football as a fight, a game pockmarked with high-energy sin and - just as inevitably - stretches of tedium.
How aggressively will O'Neill gamble?
Will he keep faith with a striker who has gone 272 days without a goal? Will he ask a 35-year-old playmaker to start his second game in four days? Will he go with a holding midfielder considered a jack of all trades and master of none?
No question, qualification for a World Cup play-off would brighten the sense of renewal around O'Neill, Keane and the extended management team. But they'll know too that it's a fine line they tread now. This, after all, is Wales in the opposite corner. Wales without their best player.
So who will speak loudest in the dressing-room? Keane has articulated a belief that international football management is, above all, about accentuating positives. "When you're with players at a club, you start looking at what they're not good at" he said in his second book. "With an international squad, you have to look at the players positively."
Needle or not, tonight they must..