Ireland 21 Days combo train/car rental

You seem to have a good start on a plan. I did my trip entirely by train/bus but it would have been nice to have a car at times. I used Dublin, Killarney, Galway, and Belfast for longer stays of four nights each. Then I made short stops in Kinsale (Charles Fort), Kilkenny, and Athlone(to get out to Clonmacnoise). In order to get to see Rock of Cashel I had to spend a day making a complicated bus trek from Cork to Cashel to Kilkenny, so that would be an area you might want to have the car if that is something you want to see ( though there may be a day tour from Cork). The key is to try not to backtrack too much. If you want to minimize the use of the car, then you may have to backtrack at some point as the train routes fan out from Dublin. If, however, you are willing to rent a car in Killarney and drive to Galway, this will save you some time. So spend your first few days in Dublin, and you may be able to find a day tour that will cover what you want to see in Wicklow, then take the train to Cork, from there to Killarney. From Killarney rent the car to get to Dingle, the drive up the west coast to Galway. Drop the car in Galway and take the train to Belfast and fly home from there. If you would rather have a car for Wicklow you could take the train to Kilkenny, only an hour, and rent a car from there. Probably easier than getting a car in Dublin. Then use the car for Wicklow and sights like Rock of Cashel that are difficult by train, and drop the car in Cork.

I would start with identifying exactly what you want to see, and determine if you can get there without a car ( maybe by using a day tour). Doing a circle route with minimal backtracking would be the most efficient. I’m sure far more knowledgeable people will chime in.