21 days Scotland and Ireland...Ireland 3 (?) days

If you have not driven in the UK before, you will likely find it to be quite challenging. Public transportation is not the best in Ireland, so you would need to drive there, but I would consider other transportation in Scotland.

@cala

Why specifically?

I know driver on the right, but friends who have been to Scotland, advise me that after a short period of adjustment, it should be fine. I figured three days on the back roads over to Letterkenny/Cleeselough, I might have the hang of it.

Double check car rental in Ireland for drivers over 70 years of age. Just something to be considering.

The short answer is no - you really can't do what you propose to do in Ireland in only 3 days ... and you'd wear yourselves out trying.
Creeselough is about a 5 hour drive from Dublin. Drive times in Ireland, especially in rural Ireland, are much longer that some of the mapping apps would have you believe.
A possible alternative would be to do the Scotland portion of your trip first, then fly from Glasgow to Donegal, pick up a car there for your roots quest, then make your way back to Dublin at your leisure. Donegal is one of the prettiest parts of Ireland but it is remote. Suggest devoting at least a few days just exploring the charms of the region.
A one-way drop charge, as long as you pick up and drop off in the Republic, is pretty minimal. You could drop the car at Dublin airport before your return flight to the US with minimum hassle.

@Robert,

Yeah, we thought of that but what would the destination in Donegal be if we flew from say, Glasgow and how much would it cost?

Flying home from Dublin would be no more hassle than flying from Glasgow I suspect. Overall, I like the idea, however.

Thanks for your advice, BTW. If we flew into Dublin and drove to Cleeselough could we fly out of Donegal to Campbelltown. The second half of my 'roots quest' is Castle Sween on Loch Sween.

And just out of curiousity, going back to the original plan (driving to Cleeselough from Dublin and back to Ballycastle, etc.), where is the hang up? What makes it unfeasible? You say driving conditions make for longer times than apparent but I kind of figured 4-5 hours to Cleeslough. Is it the stops along the way, time in Dublin or just the driving times altogether?

We had no problem driving in the UK. If you consider yourself an experienced driver; driving on the left is not that difficult. Round-abouts can be a little tricky because you have to watch the other cars changing lanes for their exit. It is best to have the driver concentrate on the merging while the "navigator" watches for the proper exit. Saving Ireland for the end the trip sounds like a good idea; using Donegal as a special ending. Since you are driving in Scotland; don't get hung up on just the "Highlands". There are plently of other scenic and historic parts of Scotland. I would recommend going at least as far South as Hadrian's Wall to see the ancient boundary. The Border Abbeys are very evocative. Look the Visit Scotland, Secret Scotland, Scottish Heritage, National Trust for Scotland and World Heritage web sites for a lot of great ideas.

@Kathleen,

Thank you. Good advice about driver and navigator. Just the sort of thing I need to hear.

Yeah, the 18 days in Scotland is another topic (in the Scotland Forum)...starting from Campbelltown we thought to travel up the Kintyre peninsula (Castle Sween) and the west coast to Oban and from there to Inverness and then back down through Falkland and Edinburgh and then over to Glasgow.

That's why I had thought Dublin and Donegal would be a good way to start. The ferry over to Campbelltown gets us off on our Scotland leg almost perfectly. Not sure how easy it would be in reverse. Got to look at the maps and think about it.

To repeat one of my initial questions... in one of his videos Rick Steve's suggested that in May booking a B&B on-the-fly so to speak shouldn't be a problem. I'm wanting a little bit of leeway to be flexible with my itinerary--not having to be at a certain place on a certain day just to get accommodations. Thoughts?

I'm seeing direct flights between Glasgow and Donegal airport (1 hour) for as little as $86 one way. It'll depend on the day of the week, etc., but suggest that you check out some of the popular search engines (google flights, skyscanner, etc.) and see what your options are.
Once you get off the modern motorways in Ireland your pace will slow dramatically. The N roads are typically 2 lane affairs where it's virtually impossible to achieve the posted speed limit unless you have a death wish. Then there's always the chance of encountering a slow moving truck, bicyclists out for an afternoon ride, or the odd herd of sheep or cattle being moved from one pastrure to another that always makes driving in rural Ireland such an adventure. Most here suggest adding another 30% to whatever drive times you see quoted on one of the mapping apps, and we've found that to be about right.
Driving on the left is not that big a deal and the learning curve is easily negotiated if you're a reasonably competent driver anyway.. I've found that the riskiest part isn't at the beginning when I'm extra careful and cautious, but a day or two into the adventure when I've relaxed a bit and start resorting to muscle memory ... which of course is all wrong after years of driving on the right. I usually preview my daily routes by using the street view feature on Google maps - helps me to have seen at least portions of the road in advance, and studying turns, roundabouts, signage, etc. helps me a lot before I approach them at speed. Might work for you too.
That's good advice about relying on your co-pilot to help you navigate, look for traffic in unfamiliar places, etc.
It doesn't look like there's an easy way to get to Campbeltown. Appears to be a good 3 or 4 hour drive from Glasgow, so it looks like you're in for a very pleasant scenic drive thru the Scottish countryside getting there and back.
Just curious, but why the ferry? Are you considering trying to take a rental car across? If so you might want to check company policies regarding taking vehicles from one island to another. Even if it's all UK (NI to Scotland) they get a little snippy about having to relocate a one-way rental back to the country of origen. At the very least it's probably cost you an arm and a leg.
You're familiar with open jaw (or multi-destination) flight plans, right?

Well like I said in my previous post Campbelltown seemed to be a good starting spot for my destinations in Scotland. it places me in easy striking distance of Loch Sween (which isn't on the main road either). And from there to Oban and Skye and Inverness, etc., ending up in Glasgow. Just thought the ferry would be easiest and fun. No we wouldn't try to take a rental car across. maybe a flight from Donegal to Campbelltown? Is there such a thing?

Where is the Donegal Airport? Donegal is a county, right? What's the town?

There is an airport at Campbeltown, and I believe there are connecting flights (via Glasgow) between it and Donegal.
Donegal is the name of the County, and there's also the town of Donegal . The airport is located in the northwest part of the county ... kind of in the middle of nowhere. Looks like it's about 45 mins. from Creeselough.

Re "booking on the fly": the main risk you run is whether the room you want at the price you want will be available. I'm sure you would find a room in Dublin. But it might not be the room you were hoping for at a price you're willing to pay. I don't know anything about Cleeslough or Ballycastle. If they are small towns, or not well-visited towns, there may not be that many rooms available to begin with.

I should probably post this in the Scotland forum but . . . About Campbeltown and the Kintyre peninsula . . . I love that part of Scotland but never managed to get all the way to Campbeltown. (Note it's spelled with just one "l".) I understand it was a famous distillery town but has seen better days. I would have liked to visit the Linda McCartney statue and see if I could chat with any townspeople who knew the McCartneys. The coastal A83 along the Sound of Jura is peaceful and scenic. That takes you to Tarbert, a nice still-working fishing village. The castle of Robert the Bruce is on the hill up above and you can walk across the isthmus from there if you're energetic. It has a helpful TI office.

NOTE Tarbert is the site of a major regatta & golf tournament each year in May, so all the accommodations for miles around are fully booked during that week.

Just north of Tarbert on Loch Fyne is the Stonefield Castle Hotel, which was never really a castle but a gentleman's country house, kind of like a small-scale Downton Abbey, with a commanding view of the loch. Then further up is Lochgilphead with easy access to Kilmartin Glen. Finally, near the head of the loch is Inveraray with its castle, historical jail, lovely Georgian architecture. Nearby are the Auchindrain Township open-air museum and Argyll Adventure (horseback riding).

I hope this gives you some ideas of how to fill your time in Scotland!

This sure has been helpful although a little bit alarming...or maybe i should say daunting.

So how does this sound? Flying from Glasgow to Londonderry and driving from there to Creeslough and Castle Doe on Sheephaven Bay. Then driving to Dublin stopping at Beltany Circle and Bru na Boinne. Stay a day in Dublin visiting the Cathedral and shops and flying home on the third day.

Do-able?

The only issue would be with the rental car. If you pick it up upon landing in Londonderry and then drop it in Dublin that qualifies as a one-way international rental for which there will be a steep surcharge. Might still be worth it to you for the convenience involved but do check the fine print before you commit.
For info, there's an Enterprise desk at the airport in Donegal. Renting from there and dropping in Dublin wouldn't trigger the surcharge since it's all within the Republic.

Oh! I hadn't thought of that!

Derry is closer to Creeslough than Donegal, so I thought that would be the best option. But Derry is right on the border isn't it? Too bad there isn't a rental just into Donegal.

[sigh] It's never easy...

Actually, Donegal airport is closer to Creeselough than Derry - though the difference is minimal.
Remember that Donegal airport is not co-located with Donegal town - it's a 1+30 drive from one to the other.
Hang in there. You have a year to sort this out.

...arriving in Dublin after a nine hour flight, ...spend the night in Dublin..

Good thinking. After a flight that long, being jet lagged, driving on the other side, and being on new turf could be an accident waiting to happen. Some are not bother by those things while others (self included) shouldn't be allowed near sharp instruments.

@Robert

Thank you. That is very helpful. You've been very helpful. Hard to get my head around $200.00+ per day rental and a $1500.00 deposit.

You need to keep shopping around for the best overall package (flights + rental car) that combines comfort and personal convenience with cost. $200 per day for the car is pretty outrageous - am sure you can do a lot better than that with a little digging.
Just a thought, but given your specific interest in Doe Castle an alternative to driving yourselves (in Ireland anyway) might be to fly in to Derry and just book a car and driver for the day - ie a personal tour guide who can handle all the details of what is essentially a day trip for you. Would certainly ease your logistical challenge and might cost less to boot.
Derry is a fascinating city, and if your ancestry traces to Donegal then it's likely that your people emigrated from Derry anyway, which might help further personalize a visit to the city.
You'd still need to make your way back to Dublin via public transport, but bus and train connections are cheap and efficient. From Dublin you could then book day trips north to Newgrange & the Boyne Valley and/or south to Glendalough, thereby accomplishing your Ireland objectives while never having to worry about the hassle of a car. Just a thought.