If we want to have a home base in Dublin,Galway, cork, 3 days each, what is the best way to have day excursions ?
I'm not sure if I'm answering your question, but I would recommend renting a car for all of your time outside of Dublin. In Dublin you don't want a car, and if you only have 3 days there, you can easily spend 3 days enjoying the sights of Dublin without taking any day excursions out of the city. There are, however, bus tours out of Dublin to some of the nearby popular destinations.
If this doesn't answer what you were asking, try rephrasing your question.
wouldn't it depend on where you wanted to go?