Tower Of London is back out again quickly in the Friends Of The Curragh Irish Cesarewitch having finished fourth in the St Leger at Doncaster last week.
The three-year-old features in a maximum field of 30 and he will be ridden by Ryan Moore, who jets back from Australia overnight to take the ride.
Tower Of London, a brother to Irish Derby and St Leger winner Capri, already has one big handicap win on his record in the Ulster Derby before he stepped up in class to be second in the Bahrain Trophy.
While no match for stablemate Continuous on Town Moor, he finished only a length behind the runner-up Arrest.
O’Brien won the race with a three-year-old last season when Waterville came from last to first to win in devastating style.
O’Brien said: “It’s a little bit quick for him to be coming back, but it is an important race and we thought it was worth taking the chance with him.
“This horse is rated much higher than Waterville when he won (last year), so that means he’s obviously got a lot more weight than Waterville had.
“He ran a good race in the St Leger, he ran well in it and he wasn’t beaten too far.”
Numerically the race is dominated by National Hunt trainers – principally the champion Willie Mullins who runs six.
Top weight Jackfinbar, the mare Echoes In Rain, Stratum, Mt Leinster – who is the mount of Rachael Blackmore – Lot Of Joy and M C Muldoon.
James Fanshawe sends over recent Goodwood winner Novel Legend, while Adrian Keatley runs Legendary Day, who won the Mallard at Doncaster last week.
Emmet Mullins holds a strong hand with Teed Up and Cheltenham Festival winner The Shunter, with Galway Hurdle third My Mate Mozzie representing Gavin Cromwell.