Welcome to my forum. You have plenty of time for planning.
It is usually quicker to fly between Scotland/England and Ireland. If you are thinking of taking a car from Scotland/England to Ireland on a ferry you will need permission from the car rental company. It is usually not what tourists do. Instead natives of Great Britain ferry to and from Ireland with cars that they own.
You can comparison shop by getting prices for flights versus ferries. Look at my Transport page for links.
Hope that gets you started. Let us know if you have any other questions as your trip progresses.