Wexford-born D'Arcy, at 31 a year younger than Dubliner O'Driscoll, has played 60 tests.
It sounds like suit and tie, bacon and eggs - you shouldn't have one without the other - so long have they been integral figures in the Irish side.
So what makes the partnership tick? D'Arcy chuckled yesterday at the notion of anything telepathic at play.
Even so, after that length of time a certain degree of instinct must surely take over.
"As with any partnership - you see it with No9 and 10, or the back row - if you play with somebody long enough you get to read their movements, their fades, the tells they have," D'Arcy said.
"Then it just becomes like practice. You train and play with somebody enough you get to trust them that extra bit."
O'Driscoll has been among the game's great backs, a sharp, incisive attacker, resolute defender and with an eye for an opportunity only the finest possess.
If D'Arcy has been the less flashy of the pair, he's been a top class foil, a hard-running, determined footballer good enough to have been on the last two Lions tours.
But as for a secret communication method - sly wink, a tug on the left ear to indicate a planned move - D'Arcy puts the relationship in more straightforward terms.
"There's little things that just happen on the field. Sometimes you can't explain it; sometimes it comes straight off the training field."
In this case, practice, trust and familiarity have bred a cornerstone of Irish rugby over the past decade.
Having seen off the emotion-laden challenge of the United States last Sunday night - September 11 - their first big challenge comes at Eden Park on Saturday against Australia.
Irishmen at the top table at their media session yesterday made plain their admiration of Australia's attacking prowess, and the size of the challenge facing them.
But if that sounds like they were actively pursuing the underdog status, it wasn't quite as simple as that. Praise was tempered with self-belief.
Backs coach Alan Gaffney said it was pointless singling out any one Wallaby for special attention.
"I think [halfback] Will Genia is absolutely outstanding. He is some player. But right across the board, they've got so much potency in the group," he said.
D'Arcy didn't come this far to try and sneak into the quarter-finals as pool C runners-up.
"We can prepare ourselves as best we can, but only if you don't believe you're already on the back foot," he said. "Belief is massive, belief in the guy on your left and right.
"Australia are a very good team, you'd be foolish to say anything [else] but there's no point in me sitting here saying I don't think we can beat them.
"You can't put them on a pedestal. We have to look at them like every other team we've ever faced - then just go out and beat them."
Of their last 10 meetings, Ireland have won two and drawn one, so Australia's impressive start against Italy notwithstanding, the Irish are certainly not without hope.
Ireland v Australia
Saturday, 8.30pm, Eden Park
