The cover mentioned is a little out of date in naming, but this is what you want
http://www.theaa.com/european-breakdown-cover/faqs.html
Originally Posted by TheAA
What is AA Five Star cover?
This is the former name for European Breakdown Cover. It was renamed in 2005. Find out more about the history of AA Five Star. The AA also offers a car warranty, which is known as Five Star, but this is a completely different type of cover.
You can buy it here:
http://www.theaa.com/breakdown-cover...down-cover.jsp
(The AA is one of the UK's breakdown companies so if the car broke down they would tow it back to a Hertz location, I suspect the Irish Hertz doesn't have a deal to cover the car in Scotland)
As for the pricing, that huge amount is probably as a result of the one-way intra-country drop off fee. The only way to reduce that would be to do 2 rentals. Personally I think your route will be very nice, and a good holiday/trip, but it will be more expensive than for example driving Dublin-Belfast and then catching a cheap flight with flybe.com to Edinburgh/Glasgow and picking up a new hire car at the airport.
Alternatively there is a Hertz directly at the Ferry terminal in Stranraer, not sure if it would be cheaper to do two one-ways, but it could be.
Hope you enjoy your trip.