Transportation between London - Wales - Southern Ireland - Northern Ireland - Scotland

Your second solution might work; except I suggest keep Irish car rentals separate from Scottish rentals. The insurance, fees and return policies for the 2 countries might be a problem. Check the cost of cars on Ferries. In any case, turn in car in Ireland and go passenger. When you take Ferry to Scotland you have a Bus Transfer which takes you to Glasgow. Rent and return cars in Glasgow; you'll have a much better choice and service at the car rentals near the airport. Check costs of flights to Glasgow from Ireland if time vs money is what you want. I'm sure others will post in more detail.

You can take a hire car out of the UK (and return it) so long as you're prepared to pay the rental firm’s applicable fees. If you looked at those and found them prohibitive what are you options?

Well you could drop the car at an airport (e.g. Cardiff, Bristol, Manchester) fly to Ireland, then hire another car. Then you’d have an Irish rental car that you’d have to return within Ireland before heading to Northern Ireland, where you would get another car (which you’d have to return before flying to Scotland). Then pick up another car in Scotland. All-in-all quite awkward, though possible.

It might be easier to do the England, Wales, Scotland leg first before flying to Ireland/Northern Ireland.

Are you really keen on riding the ferries? Or did you plan on ferries because the geographic "as the crow flies" distances seemed logical? If the latter, I would suggest you rethink the whole idea. You might be better off touring each island separately: 1. Great Britain (England, Wales, Scotland) and 2. the island of Ireland (Republic of Ireland and Northern Ireland).

You can fly between the two islands more quickly than taking ferries, and the cost is likely to comparable even with your party of 8 people. There are frequent flights to Dublin, Shannon, and Belfast from the major airports in England and Scotland. You can rent vehicles at any of these airports.

By the way, did you mention how many days your trip will be? For a party of 8, you'll only move at the pace of the slowest member. You've outlined a lot of territory to cover -- what I would consider at least 3 weeks' worth if not 4 weeks.

My thought was the same as epltd's: Sounds like a lot of territory for what I'm guessing is the time available. I fear you're going to be spending more time loading up the cars and driving to the next stop than you'll actually be sightseeing.

Exactly how many nights will you be spending in Europe, not counting the night on the plane?

Thank you all for the replies. We'll be in Europe for 18 nights, not counting flights to & from. We got a great price on flights from NYC, which is why we're starting in London and flying home out of Glasgow. And we're staying with friends in Cork for a few days, but want to spend majority of the time in Northern Ireland & Scotland.

We thought initially that ferries would be the easy way to go with car rental, but hadn't considered the obstacles & extra expense with changing countries with the rental. We will definitely look into flights to compare.

And, epltd and acraven, to your points, while our group are all pretty efficient travelers, we're also considering eliminating Edinburgh as a stop rather than trying to blow through that in 1.5 days to simplify the itinerary. I think the scenery and outdoor activities of the Highlands/western half of Scotland is what we mostly want to see. At least on this trip.

Thanks again for the insight!

First, a gentle reminder that you are talking about the Republic of Ireland, not Southern Ireland.

I’ve never traveled with a group of 8 and I think that dynamic is a huge factor in your transport choices.
I suggest pricing out several options to get a sense of both the time and cost involved, then look for the best value that balances cost/time.

I agree that really narrowing your focus to “must see” locations is a great idea.

Rent separate vehicles in each country. Otherwise you will have international drop-off fees. Take the ferry from Belfast to Cairnryan as passengers, get your Scotland car at Cairnryan. We did an Ireland-Scotland trip a couple years ago, flying into DUB and renting a car there. We returned the car to DUB and took the bus to Belfast for the ferry, got new cars in Scotland. Seemed to be the least hassle and expense that way.