William Irwin (born 20 May 1968 in Niagara Falls, Ontario, nicknamed "Billy the Kid", is a retired Canadian amateur lightweight and professional light/light welter/welterweight boxer of the 1990s and 2000s who as an amateur won a bronze medal in the Boxing at the 1991 Pan American Games in Havana, Cuba, losing to eventual silver medal winner Patrice Brooks of the United States, represented Canada at the 1991 World Amateur Boxing Championships in Sydney, Australia losing to Julien Lorcy of France, and represented Canada at the 1992 Summer Olympics in Barcelona, Spain, he was stopped in the second round of the lightweight division (– 60 kg) by Ronald Chavez of the Philippines, and as a professional won the Canadian Professional Boxing Council (CPBC) lightweight title, Canada lightweight title, International Boxing Organization (IBO) lightweight title, World Boxing Council (WBC) Continental Americas lightweight title, and Commonwealth lightweight title, and was a challenger for the International Boxing Federation (IBF) lightweight title against Paul Spadafora, and World Boxing Association (WBA) World lightweight title against Juan Díaz, his professional fighting weight varied from , i.e. lightweight to , i.e. welterweight.