Ferry bookings work in the same way as flight tickets these days, the actual fare depends on the date and time of the booking, and changes in time.
To check the fare that is of interest you need to go to the ferry company website, e.g. Stena Line, enter the dates, press continue booking, enter the number of passengers and the size of the car, and then you will have all possible ferry times with the combined fare---which you can book.
Unfortunately you will have to do similar steps to check the prices of all the other means of transport.
If I have to guess, I would say that the ferry only makes sense if you already own your vehicle. Otherwise, it will make more sense to get to Dublin on the public transport, and rent your car there.