They have been side by side ever since, running a centre partnership which has stood the test of time over 15 rugged seasons at the highest level. Between them they have appeared in more than 200 matches for Ireland and more than twice as many for their native province.
That they are still good enough, not for any old team but one with serious designs on finishing the season as champions of Europe for the fourth time in five years, says everything about their enduring defiance of the ageing process.
For sheer longevity in the professional era, no other midfield pair has come even close to matching them. O'Driscoll may be coming up 35 and his marginally junior partner 34 but even after all these years, they are still very definitely in the business of winning silverware.
Both went the full distance in Galway last weekend when D'Arcy's first try of the RaboDirect PRO12 campaign helped the defending champions overcome Connacht. Now they switch their attention to Europe and what may turn out to be the last performance of their double act on French soil.
Castres on Sunday at the Stade Pierre Antoine where the French champions have not been beaten all season is a match Leinster must win if they are to qualify for the last eight and, more crucially, a home quarter-final. The damage inflicted at the Aviva Stadium before Christmas when Northampton achieved their astounding 42-point swing leaves the triple former champions no room for error.
Leinster cannot afford a repeat of what happened when they were last in Castres five seasons ago, a three-point defeat that cost them home advantage at the knock-out stage, not that it mattered unduly. They recovered to win their first Heineken title, beating Harlequins in London, Munster at Croke Park and, finally, Leicester at Murrayfield.
Castres, third in the Top 14 on the strength of sweeping all before them at home save for a 22-22 draw against Clermont, have also seen Northampton and the Ospreys off their premises.
On the debit side, pointless trips to Dublin in October when Jack McGrath accounted for the only try and to Swansea before Christmas have effectively eliminated Castres from quarter-final contention.
The penultimate round of the pool competition kicks off with another Franco-Irish affair in Belfast on Friday night. Ulster, the only team to have won all four European ties this season, will be expected to make it five-out-of-five by completing the double over Montpellier.
French clubs would cheerfully give Ravenhill a wide berth, especially on a Friday night. Losing there has become an annual event over the last eight seasons - Castres last year, Clermont the year before, Biarritz the year before that, Stade Francais in successive seasons, Bourgoin and, most spectacularly of all, Toulouse, thrashed by 27 points seven years back.
In twelve seasons, only one team, Biarritz, has won a Heineken Cup tie in Belfast. Normal service having been resumed there against Munster last Friday after the previous week's sobering experience in Dublin, Ulster know that nothing less than a home win will be good enough to stay ahead of Leicester.
Despite losing at Ravenhill in the opening round, the Tigers' winning run since has kept them within striking distance of the leaders.
Should they emulate Ulster's achievement before Christmas with a bonus-point win over Treviso in Italy, the stage will be set for a momentous decider at Welford Road next week.
Munster, the third RaboDirect PRO12 team to be found among the current six pool leaders, will be ready, if necessary, to go however long it takes at Kingsholm on Saturday. Nobody tends to be more dangerous when the clock turns red as Perpignan will testify to their cost.
When Munster were last at Gloucester, for a one-sided quarter-final six years ago, they went on to win the whole caboodle. The English club, awaiting heavyweight front-row reinforcements for next season in the Ulster All Black John Afoa and the Welsh Lion Richard Hibbard, may already have missed the European boat to the last eight.
Seven days after beating Edinburgh comfortably at Murrayfield, they lost the home return 24 hours before Tommy O'Donnell and JJ Hanrahan sprang their Houdini act on Perpignan.
Munster know all about Edinburgh's capacity for producing the unexpected. The Scots did for them in the opening round, a home win made all the more unlikely given the fact that they went into it having lost all five matches in the Pro 12.
That Cardiff Blues are still in contention for one of the two best runners-up slots is a measure of their commendably rapid recovery from the catastrophic torment of their first-half capitulation at Exeter three months ago.
All Phil Davies' team has to do now is follow their Arms Park win over Toulon with another down on the Mediterranean on Saturday afternoon. It would be a tall enough order with a fit Sam Warburton at the head of a full-strength Blues team never mind one hit by injuries, not that the defending champions have covered themselves in glory of late.
They have been beaten in three of their last four matches in the Top 14, losing since Christmas to Racing Metro in Paris and at home to Grenoble, without managing a try in either.
The top eight as they stand after four rounds:
1 Ulster (18 points, 13 tries)
2 Toulouse (15 points, 15 tries)
3 Clermont (15 points, 11 tries, +42)
4 Toulon (15 points, 11 tries, +36)
5 Munster (14 points, 11 tries)
6 Leinster (13 points, 8 tries)
7 Saracens (15 points, 17 tries)
8 Leicester (15 points, 11 tries)
For the record: Gordon D'Arcy's European debut, Stradey Park, September 19, 1998.
Llanelli 27, Leinster 33.
Leinster:
Kevin Nowlan; Gordon D'Arcy, Martin Ridge, Shane Horgan, Denis Hickie; Alan McGowan, Derek Hegarty; Henry Hurley, Shane Byrne, Emmet Byrne; Pat Holden, Gabriel Fulcher; Trevor Brennan, Victor Costello, Craig Brownlie.
Llanelli:
Darril Williams; Wayne Proctor, Neil Boobyer, Nigel Davies, Garan Evans; Stephen Jones, Aled Thomas; Phil Booth, Robin McBryde, Sean Gale; Chris Waytt, Mike Voyle; David Hodges, Iwan Jones, Hywel Jenkins.
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