Winning in London and Paris during the course of the same Championship has not been done by any Irish team since Tom Kiernan's in 1972. They beat France at Stade Colombes 14-9 and England at Twickenham a fortnight later 16-12, only to be denied a crack at completing the Grand Slam at home because their opponents failed to turn up.
First Scotland, then Wales chose not to go to Dublin because of The Troubles in Northern Ireland. No amount of pleading by a six-man Irish Rugby Union delegation sent to Edinburgh and Cardiff made any difference.
The Scottish and Welsh refusal to travel prevented some of Ireland's greatest players from converting a rare away double into the equal rarity of a Slam.
Until Kiernan's unlucky team did it in the early Seventies, the London-Paris double had not been done since 1948, the momentous year when Scotland and Wales did turn up after Jack Kyle & Co had won the opening matches in France and England.
Having eased past Scotland in the opening round by 22 points and followed that by going one better against Wales in knocking the champions off their pedestal, Ireland go to Twickenham next week as the team to beat. Their coach and captain know how it feels to win there.
Joe Schmidt did it in style albeit at the expense of Irish rather than English opposition, presiding over Leinster's exhilarating brand of Total Rugby which eventually engulfed Ulster during the European Cup final two years ago.
In his first season as an international coach, the New Zealander goes back there with a squad in serious contention for what would be only Ireland's second title since the mid-Eighties.
Paul O'Connell has seen Ireland win on three of their last five visits to Twickenham, the first, most famously, ten years ago when Billy Williams' old cabbage patch really was a fortress.
England, with their newly-won World Cup on display, had not lost there for five years until the Irish took it by storm thanks to Girvan Dempsey's try and five Ronan O'Gara goals.
Three of that team will expect to be there again this time - O'Connell, Brian O'Driscoll, still peerless at 35, and his perennial midfield partner Gordon D'Arcy, a comparative slip of a lad at 34.
Home after losing unluckily in Paris and winning comfortably in Edinburgh, England see Ireland as a severe test of how far they have come on the road to recovery from their disintegration in Cardiff 11 months ago. Ireland see England in much the same light.
No sooner had the Welsh victory roar subsided than O'Connell had acknowledged the point: "The competition gets a whole lot harder from here. Twickenham's going to be a massive step up. It doesn't matter how many times you've done it before, a win at Twickenham is massive."
Ireland under Schmidt are in the business of victory on a monumental scale as Wales will testify. In the course of outplaying the holders, the Irish torpedo hit them amidships with enough force to leave a gaping hole in their avowed aim of sailing home to the first hat-trick of RBS 6 Nations titles.
Warren Gatland had not experienced a defeat like it in Championship terms as Wales head coach, worse even than their last beating on the road, 28-9 in Paris three years ago.
Wales had not been restricted to a single penalty goal in the tournament since their first match in the RBS 6 Nations, against France in Cardiff in February 2000.
No country tends to hold a more heated or protracted inquest into national defeat than Wales. That they must wait until Friday week when France come to the Millennium Stadium means this one will go on longer than usual.
Family doubles come in all shapes and sizes but one went without mention last weekend when it would have justified a fanfare. Rarely has history repeated itself quite as neatly as in the 48th minute at the Stade de France last Sunday.
Hugo Bonneval, the French left wing, scored a try on debut. His father Eric, in the same No.11 jersey, had done exactly the same 30 years earlier, against the All Blacks in Auckland.
The romantic notion that Scotland would somehow defy the odds and come up with a family double of their own against England turned out to be a forlorn one.
In 1984, a couple of months before Bonneval, senior, made his bow in New Zealand, Scotland had won the Calcutta Cup with a decisive 18-6 win over their southern neighbours en route to winning an even more glittering prize, the Grand Slam.
Roy Laidlaw, the Lion from Jedbergh, was a permanent fixture at scrum-half. The current No. 9, his nephew Greig, learnt the hard way that an action replay would not be possible but then uncle Roy did play behind some pack of forwards - Jim Aitken, Colin Deans, Iain 'The Bear' Milne, Bill Cuthbertson-Alister Campbell, Alan Tomes, Jim Calder, Iain Paxton and David Leslie, the most fearless of wing forwards for whom words are difficult to find to do him full justice.
Suffice to say that no player can ever have put his body on the line more readily in his country's cause.
The red cards flashed at substitute props Michele Rizzo (Italy) and Rabah Slimani (France) in Paris evoked memories of the last double Championship sending-off, in the same city more than two decades ago.
France-England at the Parc des Princes on Saturday, February 15, 1992 made such a mockery of the Entente Cordiale that the Ulsterman charged with enforcing law and order did so by dispatching two-thirds of the home team's front row.
Referee Stephen Hilditch sent both off for early baths. Gregoire Lascube, a member of the Gendarmerie, was the first to go for allegedly doing to PC Martin Bayfield what the then humble Bedfordshire bobbie, likened to 'Fred Astaire doing a tap dance on my back'.
No sooner had the France loosehead been given his marching orders than he found a tearful hooker Vincent Moscato marching in his footsteps after an illegal tete-a-tete with England's unflappable tighthead, Jeff Probyn.
In the mayhem that followed, the France captain, Jean-Francois Tordo, might easily have provoked Hilditch into pointing him to the dressing-room before the end of a scrap that England won by the proverbial boulevard.
The referee, then headmaster of a school in Belfast, was given a police escort off the field in the furious aftermath of the second double dismissal in the Championship.
Norman Sansom, the Scottish referee responsible for the first, at Cardiff Arms Park in January 1977, blew for no-side and made his own unflustered way back to the dressing-room without requiring any protection from the long arm of the law.
He sent the Wales lock Geoff Wheel off for flattening Ulster's Lion of a flanker Stewart McKinney and another Irish Lion, Willie Duggan, hot on McKinney's heels for taking retaliatory action. Sansom, without recourse way back then to any such thing as a video referee, relied on the evidence of his own eyes, likewise Hilditch in Paris 15 years later.
Team of the weekend:
Best of the rest:
M Brown (England); Y Huget (France), L Burrell (England), W Fofana (France), J May (England); J Plisson (France), D Care (England); A De Marchi (Italy), D Szarzewski (France), D Cole (England); J Launchbury (England), J Furno (Italy); T Wood (England), C Robshaw (England), L Picamoles (France).