Record-breaking Irish trainer Aidan O'Brien's horse Australia has taken out the Irish Derby, leaving O'Brien to hit back at critics who claim Ireland's premier classic has become virtually a benefit race for his stable.

Australia led an O'Brien trifecta in the race, his fifth 1-2-3 in the Irish Derby and his 11th winner overall.

Australia also won the Epsom Derby earlier this month.

O'Brien's other two runners, Kingfisher and Orchestra, finished second and third at the Irish Derby on Saturday.

It was the second time in the last three runnings that only five runners had competed in the race, worth more than 700,000 euros ($A1.05 million).

O'Brien is adamant the race which, prior to his domination, often saw fields in the high teens go to the post, is still a championship as indeed is the Irish Champion Stakes run in September and which is Australia's next target.

"They are great races and have stood the test of time and are top-quality European races," he told At The Races TV channel.

"Yesterday's Irish Derby had three Derby trial winners in it (Kingfisher, Orchestra and Fascinating Rock) and Australia ran in the Guineas.

"The quality of the race is a lot higher than everyone thinks.

"If you look back at the Champion Stakes at Leopardstown there are very few races in Europe that if you add the ratings up of the horses who run in it would better it," added O'Brien.