Time allocation in Dublin/Car rental/transportation to Scotland

We returned mid May from Ireland. I would suggest spending your time in Dublin before hitting the road. Many people love Dublin, however we were ready to go after a day.
I would suggest picking up the rental car at a city location to avoid some of the tariffs and taxes airport rentals are famous for.
Ireland rental car customers are required to take out CDW coverage. You wouldn't want to take a car on a ferry off the island. With budget airlines, flying to Scotland is preferable.
We had a Renault diesel SUV on our trip, and were thankful it got 50 mpg with fuel @$6.05 per U.S. gallon. I have driven all over the world, and found off the beaten path Irish roads to be the most difficult to drive. Only skilled drivers should undertake Ireland by auto.

We recently traveled 28 days in Ireland. Great trip. There is plenty to do for 18 days. Dublin is good for 2-3 days depending on how much you like cities. Best at beginning to get over your jet lag. Rick Steves has the information on inexpensive direct airport buses into Dublin. Don’t drive in Dublin. We spent three nights in Dublin then took the bus back to the airport and picked up a rental car. Compact manual transmission with no extra insurance. I enjoyed driving on the narrow one lane roads and returned the car without any scratches or dents. If you must go to Scotland get a different car there.