Tomba, who paid 10.6-1 on the pari-mutuel, hit the front approaching the final furlong and beat Charge D'Affaires by two lengths with Keos, the favourite, back in third.
Richard Hannon's Andreyev was sixth while John Gosden's Muhtathir finished last of the nine starters.
Fallon said: "He finished third in this race last year and people said he did not stay but he's a year older and that much stronger. I was travelling really well today and I felt confident some way out."
Silver Patriarch provided John Dunlop with a third success in the Group One Gran Premio del Jockey Club in Milan. The odds-on chance, ridden by Pat Eddery, beat Paul Cole's Posidonas by three quarters of a length.
Imperial Call, the winner of the 1996 Cheltenham Gold Cup, made a successful return to action at Limerick. Now trained by Peter Hurley, following the retirement of Fergie Sutherland, the nine-year-old had to give 2st to all his eight rivals, but the 6-4 favourite was always going well and beat Tell The Nipper by a length.