The judge found the Northern Irishman and Gabriele Moratti, the son of a former Milan mayor, guilty of "mutual injury" during the incident in the VIP section of the Hollywood nightclub in the northern Italian city in 2008.
Each had accused the other of being the aggressor and using a glass in the brawl.
Both men were sentenced to six months in prison. But the sentences can be appealed, and if confirmed would probably be suspended so neither are likely to end up going to jail, lawyers for both said.
The lawyers disagreed on whether the fight had occurred because the former racing driver made contact with a former girlfriend of Moratti.
Irvine, aged 48 and from County Down, drove for Jordan, Ferrari and Jaguar before retiring from the sport in 2003.
Moratti, 35, is a director at energy company Saras and is well known in Italy for being fined 49,000 euros ($66,600) for illegally converting a warehouse to a luxury home inspired by Batman, dubbed the "Bathouse" by the media.
($1 = 0.7361 euros)
(Reporting by Naomi O'Leary, editing by Alister Doyle)
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