Sinn Fein standing in free and fair elections doesn't fit any known definition of occupation.
Well...the native Irish, have now, got equal rights to the unionist, but they still live within an artificial state drawn up by the British to give a majority to the British Unionist living in the six counties.
So it's still occupation.
Poland was occupied by Russia; so what!! if there wasn't a gigantic Russian Army presence throughout; Did the Russian not wipe out the remainder of the elite after the Germans. To install a totalitarian regime in a headless country, doesn't require a big army after the initial invasion.