But Gabriele Moratti, son of the now former mayor Letizia, denied the charge, claiming a drunk Irvine in fact attacked him with a glass.
Reports suggested the fight was over a text message Irvine, who retired from F1 after the 2002 season, sent to Moratti's girlfriend.
In 2011, it emerged that the case would head to court, after negotiations for a settlement collapsed.
Now, on Thursday in Milan, the judge found 48-year-old Ulsterman Irvine guilty of "mutual injury", and Moratti was given the same six-month jail sentence.
But the Telegraph quoted Moratti's lawyer as saying both sentences would probably be suspended after an appeal.
And AFP news agency said all Italian jail sentences under two years are automatically suspended.