political science focuses on institution's influence on peoples' behavior(how to act under certain institutional arrangement). For example, the policy coming from an independent central bank would be different from a dependent one, because the people under this institutional arrangement have different best strategic behavior course. So political science observes and finds out the laws of the relationship between the intuition and individual behaviors/economic performance(political economy)/……
IR as a subfield of political science view the international world as a regime, or supernational institution arrangement, and try to find out the laws of peoples’(governments’) behaviors through strategic choice under different internal(parliament, court)/external(international court, un, treaties) institutional arrangement.
I don’t know about public affairs. Maybe someone else who is familiar with this field can give a skeletal view of the field for you.