The first option that comes to mind is Galway tours
They offer a 1-day sight seeing tour from Dublin to Galway. It comes in around 45 euro per person. They have a great reputation too.
With this option available, I wouldn't even consider using a more direct route through public transport as you would be missing so many things along the way.
Hiring a car with personal driver can be quite expensive, It is calculated differently dependant on who you book with, but you are talking 350-500 euro a day.
If you want more info on the car, or anything else, just let me know.
Best of luck with it.