Rental Car Travel between Ireland and Scotland

A slightly confusing question.
Do you mean cross the land border between the Republic of Ireland and Northern Ireland (the latter is of course part of the UK)?
Or do you mean take a car ferry from Northern Ireland (Belfast) to Scotland (which does not involve crossing a border because they are both part of the UK)?
There are no direct ferries from the Republic of Ireland to Scotland.

Crossing the border from the Republic of Ireland to Northern Ireland should be no problem, but double-check with the rental company.
Putting the hire car on a ferry from Belfast to Scotland may incur extra insurance cover, but again double-check with the rental company.
In all cases if you do not return the car where you picked it up, expect extra charges.

Thanks for your very quick reply and sorry for the confusion. Yes, we wish to travel from Larne, Northern Ireland to Cairnryan, Scotland by ferry; taking the rental car on the ferry with us. We will spend time in Scotland and return to Ireland for additional sightseeing prior to our departure. Are you aware of any rental car agencies that will allow this? Our plan was to pick up and return our vehicle in Dublin.

Have you tried contacting the rental car companies you're considering, and asking them this question?

Yes, a couple. I'm hoping someone can point us in the right direction.

What did the rental companies you contacted say?

The car can be taken from Dublin to Northern Ireland but not to Scotland

It can't leave the "Isle of Ireland"

Ah, now I can see why you're asking your question.
Perhaps someone here will have used a rental car company that had different rules from those you have so far contacted.
Or maybe it will be best to do two car rentals. Let's see what people say.

I noticed that Chris F seems to have different information from what you got from the car rental companies you've contacted so far--perhaps he could re-address this aspect of your question?
As I understand it, you want to go from Northern Ireland to Scotland, which doesn't (as Chris points out) involve crossing a border since they are both part of the UK.

Obviously, ultimately the car rental company will have the last say about what they will let you do and how much it will cost--regardless of what anyone here says.

I think that you are going to find that you will need two cars. This really is not a big deal. It will definitely be cheaper than trying to take a car from Ireland to Scotland! You drop your car off I N Ireland, take the ferry to Scotland and by combo bus and train or taxi and train get to Glasgow where you can easily rent another car. .

Oops, but then if you rented the car in Ireland, you would be dropping it off in a different country.

You can drop it off in the Republic and catch a bus to the ferry. From what I've seen on these forums though, the easiest and fastest way is to bite the bullet and fly. : )

Cars rented in the Republic of Ireland will usually have the condition that they can't be taken off the island of Ireland. Sometimes taking the car into Northern Ireland has an extra fee even if returning it back in the Republic.

Cars rented in Northern Ireland can usually be driven in the rest of the UK and sometimes throughout the EEA, subject to paying an additional insurance fee.

As always there may be exceptions, but those are typical terms.

If it were me, I'd drop the car off before taking the ferry. I rearrange my trip so I dropped in Ireland (rather than Northern Ireland) to save the out of country drop fee. You can take a ferry from Dublin or other locations, depending on what works best.

You will save money by walking on to the ferry, a foot passenger is significantly cheaper.

When you arrive, rent a second car and go from there.

It may be a little more involved but I'd expect to save hundreds doing it that way, which for me is worth a little extra planning.