Car rental

Very possible but it's often best to rent a separate car for each island. Some car rentals aren't allowed to go on ferries (and ferries cost money), so check the fine print and always tell the car rental company which countries you plan to visit. Within one country, major agencies don't have any significant fee for picking up and dropping off in different locations. AutoEurope is a US consolidator that we use most of the time and Dan Dooley is a Irish company that Rick also recommends.

See more at these and related links:

https://www.ricksteves.com/travel-tips/transportation/driving-europe-tips

https://www.ricksteves.com/watch-read-listen/read/articles/driving-in-great-britain-and-ireland

You probably meant England rather than Britain. Scotland is part of Great Britain.

You will probably have to make 2 car rentals. One for Ireland/Northern Ireland and one for England/Scotland.

lilly,

I assume you won't have any problem driving on the "correct side of the road". It can be a challenge at times, especially on very narrow roads. You'll also may need to pay for very good insurance coverage, especially in Ireland. A good GPS / Map / Smartphone navigation system would also be highly advised.

I went with a friend to Sicily in 2014, and he did the driving; we drove everywhere but Palermo. Contrary to some rumors, he found most of the driving not to be difficult at all. The next year, he went to England and drove. He said it was FAR harder and more stressful than driving in Sicily. In particular, many of the roads were not only narrow, but had hedges right up against them.

If you're going to London, you will want either want to start there and pick up the car upon departing, or end there and drop off the car beforehand. If you do have to drive in London, make sure you understand how the congestion charge works (it's not expensive if you pay it right away, but it goes up quickly if you don't).

Some of the other large cities are no fun to drive in either. On the other hand, a car makes seeing the small towns and villages easier.

Britain - do you mean England?

The chances are that the car hire company won't permit you to take the car on the ferry. Renting a car in Ireland and dropping it in England will incur a huge one way drop off fee. So, hire two different vehicles.

I hope that your trip is 3 weeks or more, otherwise you are trying to fit too much in. Fly open jaw to maximise your trip.

Having a car in any city here is a huge disadvantage, from traffic congestion, trying to negotiate one way streets and the high cost of parking. Trains are the best way to go between say Edinburgh and London or any long distance travel. Cars are better if you are trying to cover rural areas or undertaking less long distance travel.