French Derby winner The Grey Gatsby has beaten his Epsom winning counterpart Australia to win the Irish Champion Stakes by a neck.

Australia came out on top the last time the two Derby winners met in the International Stakes and looked to have his measure again at Leopardstown.

But Ryan Moore exacted a final effort from his mount to launch an attack on Australia in the final furlongs.

As the others faded, the Grey Gatsby finally collared Australia, who had also taken the Irish Derby in between Epsom and the International Stakes, in the final 50 metres to take Irish racing's most prestigious prize and a cheque for 580,000 euros ($A869,956).

Winning trainer Kevin Ryan said he had feared the worst when he saw them turn for home with Australia seemingly cruising.

"Turning in I was thinking 'Oh God, he's got a lot of ground to make up', but in fairness to Ryan (Moore) he was saving ground while Australia had to go wide," Ryan said.

"He made up the ground well and when he passed one and picked up, I thought he would go very close.

"He's a horse that has taken us on a great journey, both me and the owner. He'll stay in training next year and will get stronger and better. We'll hopefully have another great year.

"He won't go to the Arc. There's a chance he could go to Ascot on Champions Day."

For Moore, who turns 31 on Thursday, the winner had proved his initial judgment of him correct.

"When he won the Dante in May I really liked him," he said.

"And when we went to France I thought he could run well but he surprised me with how well he won and I thought then this is a very good horse," he said.