So Young got the better of his stablemate Zaidpour to run out an impressive winner of the Red Mills Trial Hurdle at Gowran.
Often used as a Champion Hurdle Trial by the likes of Hardy Eustace, So Young only holds an entry in the World Hurdle at Cheltenham but showed surprising speed under David Casey.
Arnaud was flanked by the Willie Mullins pair over the second last and while Zaidpour, the 4/6 favourite, had the advantage of the rail, it was So Young which sprinted clear to win by six lengths.
"That's his first graded win but he's always promised to win one," said Mullins. "He made a bad mistake at the last when he would have gone close in the Neptune at Cheltenham and we've never been able to get him to do that again. We changed a few things at home with him and it looks like it might have paid off.
"I thought Zaidpour would have won but Paul (Townend) said he was never really travelling after he made a bad mistake. They'll both be entered at Cheltenham but there are nice prizes at home for them."
Mullins had earlier struck with Djakadam (4/7 favourite) in the Shantou at Burgage Stud Maiden Hurdle. However, a trip to Cheltenham is not on the cards for him, either.
"He's a chaser in the making but might be a bit better over hurdles than we imagined," he said. "Paul said he was idling the whole way and yet some of the other jockeys thought he'd gone a ludicrous pace."
Meanwhile, Gary Moore was left reflecting on perhaps the greatest afternoon of his training career after the heavily backed Well Refreshed completed a major across-the-card double with victory in the Betfred Grand National Trial at Haydock.
Although one of the most respected dual-purpose handlers around, Moore is not a regular winner of the big Saturday prizes and was understandably thrilled after claiming two big pots.
Stable stalwart Vino Griego landed a few hefty bets when winning at Ascot under the trainer's son Jamie, and less than half an hour later, his younger brother and conditional jockey Joshua rode the biggest winner of his burgeoning career at the Merseyside track.