Car rental concerns

Do you mean, are there concerns with crossing the "border" between the Irish Republic and Northern Ireland (which is aprt of the UK)? Today, not really - the "border" is all but transparent. After Brexit? Who knows. I wouldn't be shocked if there was a little bit of confusion immediately after the change-over (there's plenty of confusion right now as they're trying to sort out what to do about that particular border) and maybe a bit more process post-Brexit, but I'd expect things to be fairly smooth once it settles down.

Personally, I'd be more concerned about trying to coordinate a large group split between two cars, and making sure you have enough room for all passengers and their bags. Keep in mind that their definition of "mid sized" cars may not be what you think of as one. If you end up with a car big enough for 4 adults and all their bags, whatever you call it, that car may feel a bit large on small roads and skinny streets in old towns and cities.

Other than the change from kilometers to miles and differences in signage the answer is no.

I would suggest trying to get a diesel mid size SUV. We had a Renault in May, and it got great fuel mileage. Even then, the luggage space wasn't larger than 4 people could use.
We went to the southwest of Ireland, down by the Ring of Kerry. Driving was difficult on backroads with crooked roads and either hedges or walls on each side of us.
If you are depending on public transportation in Dublin, you might want to go to Google Maps and choose a place to stay with restaurants around with walking distance--on a bus line. You can click on hotels and little icons of a bed will pop up all over the place. The Hilton Airport hotel was a much better value than staying in an older B&B just north of town, and buses came by all the time. A modern shopping center was also across the street with restaurants, etc.

I did extensive research on a month long car rental for May, 2019. I was advised to make sure that your rental car company knows that you plan to go into Northern Ireland. Some of the rental car companies I checked have a surcharge for traveling across the border--others did not. And they use tracking devices so you can't sneak across.

We rented a small VW in Galway this September and drove up to Northern Ireland before turning it in in Dublin. The Republic of Ireland car's speedometer was strictly in kilometers per hour, so when we reached Northern Ireland, it required continual 1 km = 0.62 mile mental calculations, to ensure we weren't speeding or going too slow. I was surprised that speedometer didn't have a second set of numbers on the dial, in a smaller font, to accommodate driving in both parts of the island.

Also, on narrow, twisty roads as you make your way down towards Galway, other cars may be passing or turning/merging between your car and the other car, so you shouldn't plan on following each other the whole way without another car (or several) in between, and the car behind might not be viewing the back bumper of the car in front the whole time. Having a means to keep track of each other for stops or pull-offs might be helpful, if you all wind up with multiple vehicles.

The only "issue" is that you need to read the fine print in your rental car contract regarding taking the car into NI as it pertains to insurance coverage. If in doubt check with the company directly to ensure that you don't risk loss of coverage while driving in the north... and to avoid any unpleasant surprises when you collect the car at Dublin airport.

As others have said, the actual crossing of the border is pretty seamless.

Didn’t have any issue renting from Avis at Dublin airport and heading north to Northern Ireland to drive the Antrim coast, then heading back into the Republic of Ireland. While there is a remote chance Brexit could be a concern by then, I doubt that you’ll notice any changes along the border. Just remember British pounds are used in Northern Ireland and Euros in the Republic of Ireland.