Ireland, Scottish Borders, England Midlands

just make sure that wherever you get the car allows it to swim across the Irish Sea. Some companies allow the car to go on ferries, some don't, and some charge extra.

Also make sure you have a car which can change from mph to kph as it crosses into the republic.

Can we rent a car in Dublin and drop it in Belfast?

If you hire a car in Dublin you’ll have to return it to Dublin (or elsewhere in the Republic). Likewise, if you hire a car in N Ireland you’ll have to return it in N Ireland. Hiring in the south and leaving it in the north will either be prohibitively expensive or simply not allowed as you’re taking it from one country and leaving it in another.

Fly between the island of Ireland and Great Britain and hire another car in England/Scotland. Then you can pick it up in Glasgow and leave it in Manchester.

Out of curiosity I did a dummy booking for a one-way rental from Dublin to Belfast on AutoEurope for a week in June. The one-way drop charge quoted is USD $185 which I assume is pretty typical for other companies. Whether that extra cost is worth it for the convenience involved is strictly a personal call, but using the easy, direct bus from Dublin Airport to Belfast would save you money should you elect to pick up and drop off at Dublin airport.
There are no restrictions regarding an international one-way rental between the two countries that I'm aware of, but you'd need to sort out the matter of insurance to avoid any unexpected issues if you do decide to proceed that way.
Per Nigel, I doubt that any rental car company would permit taking a vehicle on the ferry, and doing so would likely void your contract and thus your insurance.

Thanks for the replies;
Re: car that changes from mph to kph, we haven't had any issues in other countries that use either and or both and Canadian cars have both, so we are familiar with translating speeds.
Ferry vs flying, after looking at where ferries from Belfast arrive in Scotland, I don't think that is a plausible option, taking a rental car on the ferry is not going to work, arriving in the small town of Cairnryan and taking public transit to a place we could get a rental car seems far too complicated.
As the plan is to be away 3+ weeks, we will not be travelling light and taking suitcases on buses or trains something we have done before; it is not a prefered choice of travel.
The drop off fee discovered by Robert is well within the budget and not dissimilar to prices we have paid in the US and Canada. I will do more research about this.
I looked at flights from Belfast to either Glasgow or Edinburgh.
Edinburgh I think is preferable, but either would work.
The only airline currently taking bookings from Belfast for June 2022 is EasyJet prices range from ~£ 25-50, and the travel time is less than I expected.
I am making the assumption that other airlines will have flights available closer to the planned dates.
Yes we would have to check in X hours ahead, it seems a much better idea than the ferry.
I think it would be better to fly from Belfast, than Dublin because it isn't an international flight to Scotland and assuming we will still be coping with Covid related issues one less set of tests and other related issues.
We are fully vaccinated and will do any other updates to vaccination that are offered.

Sounds like you're already familiar with driving on the left from previous visits to the UK, but thought I'd mention that the default rental in Ireland is typically a manual shifter - maybe not a problem for you but it can be a real hassle for the uninitiated or unaware trying to shift with their left hand while trying to sort everything else out. If you can manage a stick it'll be a lot cheaper than an automatic but for most people the extra cost for the automatic is well worth it.
Know too that most credit cards specifically exclude coverage in Ireland (the Republic that is). Some here have reported that their premium cards do work, but after numerous visits of our own and a bit of trial and error we've settled on AutoEurope as our preferred rental car provider. We find their rates to be quite competitive and by opting for their zero deductible coverage we remove the potential for any issues should we ever have the need to file a claim. The convenience and peace of mind is worth it for us - might look into it to see if it's the best option for you as well, especially given that you'll be driving across the border into NI and dropping the car in Belfast.

Thanks Robert
For us automatic transmission is worth the extra cost because it means both of us can drive without trying to remember the how tos from the last time we drove stick in addition to driving on the left.
What is the issue with credit cards and car rental?
I also looked at flights from Toronto to Belfast, they all have a layover of between 3 and 6+ hours at Gatwick, so not an option either.
The car; Budget one way Dublin to Belfast is 735.06 euros base price, I looked at Auto Europe and Eurocar but didn't make a note of the costs!
Also looked on Kayak, it seems they have some good rates, but as I have found out when you get to bottom line the total can be much more than it first appears.
And I prefer to book directly with the company as last time we didn't we ran into problems.

Yeah, opting for an automatic will save a lot of aggravation in the long run.
i don't know why Ireland is excluded from regular CC coverage (along with Italy & Israel apparently), maybe it's because of the narrowness of rural Irish roads and the proclivity of some new drivers to sideswipe the ever present rock walls that exist along country lanes. Guess it's not an unusual sight for rental car returns to be missing a side view mirror. All I know is that sorting out the insurance on most company websites can be frustrating - you have to drill down into the fine print in order to find out what's covered, what isn't, and how much everything will cost, which is why I finally gave up and started relying on AutoEurope - they de-mystify the process by explaining everything in detail right up front. We've been very happy with them over the years.
Rental car costs for bookings next summer are quite high everywhere right now. Suggest continuing your due diligence and comparing costs by shopping around between now and your proposed departure date, by which time things should have settled down a bit and prices will (hopefully) have returned to a semblance of normal.
Might add that driving in Ireland is a real pleasure. It helps that the locals are some of the most patient and forgiving drivers anywhere. You're going to love it.
Edited to add: some here have reported that their premium credit cards do provide rental car coverage in Ireland, so if you have one of those it's worth checking. Know that if you decline coverage at the counter in favor of your own insurance cover the rental car company will likely place a hold on your credit card until the vehicle is returned. The hold can be substantial - upwards of 5,000 Euros depending upon the type of vehicle and the duration of the rental.

I will check with our CC company, we do have rental car coverage on it.
On previous trips to GB, we have driven on very many narrow roads, with rock walls in Devon, Cornwall, Wales, Scotland and on our last visit in 2017, the smallish car we had booked was unavailable and we ended up with a Dodge Caravan sized van, driving that around tiny lanes on Dartmoor was an interesting experience.
And I agree British drivers are much more patient.

We've had nothing but good luck with RyanAir for flights, despite their horrible reputation for upcharging for luggage etc.

You generally won't see RyanAir come up on travel sites - I guess they won't deal on commissions.

LoganAir also flies to Dublin, Belfast, Derry and all over the UK. Sort of spendy compared to Easyjet or Ryanair, though. There seems to be some turnover in budget airlines.

Also make sure you have a car which can change from mph to kph as it crosses into the republic.

Nigel, does your car not display speed in both mph and kph?

yes, but it isn't simple. One or the other is displayed, not both at the same time. Everything is digital so it is knowing the sequence of buttons to update the speedo, (and make it stick), and then more buttons to reset the built-in sat nav.

Who wants to do that every time they cross the border?

I think in that situation having an older analogue speedo with concentric numbers in both scales would be easier... Still have to change the sat nav though, or maybe not?

After driving for 40+ years with both miles/ph and km/ph on the speedometer it becomes automatic to know both systems and translate back and forth. We have done a lot of travelling in the U.S. so we are very used to it.
I can't see a purpose in resetting a SatNav/ Gps. We never do in the U.S.

yes, but it isn't simple. One or the other is displayed, not both at the same time. Everything is digital so it is knowing the sequence of buttons to update the speedo,

Ah, I see. I have a digital speed display on my console but also an analogue one with dual mph/kph markings. I suspect most cars also have this, certainly every new car that I've rented has this arrangement so I wouldn't foresee it as much of a problem.

Mine's a 4 year old Honda Civic