A patient ride from Johnny Murtagh paid dividends as Fort Knox finished with a flourish to take the Leopardstown 2,000 Guineas Trial.
A winner of a backend Newbury maiden when trained by Richard Hannon as a juvenile, the Dubawi colt has since joined Tommy Carmody and was a 7-1 shot for this Listed contest.
Fort Knox had just one rival behind him rounding the home bend, but as the majority of the leaders began to falter, he came through with a withering run to take then prize by a length and three-quarters.
Don't Bother Me and High Octane were second and third respectively.
Favourite The United States looked decidedly one paced, while Move To Strike stopped very quickly.
Murtagh's wife, Orla, said: "We are really happy and he's in all the Guineas.
"We'll have to speak to Andrew (Tinkler, owner) and we'll see how he comes out of the race but on that ground he did it well - it is quite testing."
O'Brien's Declaration Of War made a winning return to action with a stylish display in the Heritage Stakes.
Sent off at 6-4 for his first appearance since winning the Group Three Diamond Stakes at Dundalk in early October, Declaration Of War moved sweetly on the heels of his pacemaker Windsor Palace for much of the one-mile contest.
He took over the running still going strongly halfway up the straight and found plenty in the final furlong to beat the admirabLE Bold Thady Quill by two and a half lengths.
Carmody's Irish Lincolnshire winner Sweet Lightning was favourite, but passed the post a disappointing fourth.