Itinerary suggestions for 9 nights

You'll want to return the car in the same country where you rented it, to avoid a big dropoff charge. Probably Dublin makes the most sense for this. But you can take it to the north and back.

With nine nights and four cities, your itinerary is pretty obvious, and busy. Make a big circle, clockwise or counterclockwise. Give Dublin more time unless you've been there already and want to spend more time elsewhere.

With the list of places you've identified, your itinerary is more of less sorted. Dublin, Galway, Donegal, Belfast (or vice versa). Two nights in each with a final night in Dublin to catch the flight next day.

We were only going to do one night in Belfast and Donegal. Looking for other town suggestions?

Have a look at either Portrush/Port Stewart for the seaside or Derry if you want a big thriving town.

If wanting somewhere between Donegal and Galway, then consider Sligo or Westport.

Thank you I will look up those towns