So Young got the better of his stablemate Zaidpour to run out an impressive winner of the Red Mills Trial Hurdle at Gowran Park.
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 who sprinted clear to win by six lengths.
Mullins said: "That's his first graded win but he's always promised to win one.
"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.
"We keep thinking he's a two and a half mile horse but we brought him back to two miles today.
"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 both and it may be best to keep them at home.
"They'd be going there with a little chance but both have been well beaten there even though they've run their races before."
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.
"He was unlucky the last day when he fell."