"Who are you, father?"
Translation:Cò sibhse, athair?
Do you know French? If so, think of it as the difference between 'tu' and 'vous'. Tu (thu; thusa) is used for singular and someone the same age/status; vous (sibh; sibhse) is used for more than one person, or one person if that person is older/more senior (teacher/professor/grandfather etc) Does that help?