The -ovich ending is a Slavic patronymic ending, so this would appear to denote descent from someone whose given name was Shirshun. So far I haven't found an example of this as a forename. What language? The -ovich ending is found in the surnames of Serbia, Croatia, Ukraine, Belarus, and Poland, with a variety of spellings. It was also favoured by Eastern European Jews and Gipsies. So where to start? I don't think Shirshun is Slavic. Googling Shirshun I got only American examples; googling Shirshunovich, with variations in spelling, got nothing. Do you know if it's Jewish?