How hard is it to rent a car with automatic transmission?

I had no trouble. Sometimes it can depend on the location, but in that case you typically put in a request.
Just go to your preferred rental agency and enter your details to see what options come up. I had a good experience with Europcar.

Not hard - just more expensive..
Since the default rental in Ireland is usually a manual shifter you'll need to take care while booking to specify exactly what you want.
The Dublin airport location typically has the largest selection of rental cars in the country - so you'll maximize the availability of an automatic by booking from one of the airport concessions. For info, the airport offices open early (around 5:30 AM) and close very late. The city locations have much more restrictive operating hours, though they may be a good choice if you're staying near one of them. There are a couple along South Circular Road that can be particularly attractive options if you're heading south out of the city - it's a straight shot out of town from there so no issues about driving in or near the city center.

You just need to make sure you specify an automatic when you make the reservation. Read the description carefully. Picking up at a transportation hub (airport or train station) will give you a bigger selection that at a small rental office in a small town. Make sure to reserve in advance.

Major airports are your best bet ( and major agencies , with large inventory } This coming Autumn , I will pick up at Manchester UK and drop at Edinburgh UK

I can't speak specifically to Ireland, but there is a difference between reserving an automatic and receiving one (especially, a similar sized vehicle) upon pickup. Pre-pandemic, I was offered an (automatic) huge delivery van at Frankfurt, and then had to wait an hour for a standard transmission car (smaller) to be returned and turned-around for us.

The last time I rented a car with an automatic transmission we got to the Hertz desk at the Frankfurt airport did three rounds of the "do you really want an automatic" dance until I explained to the guy at the counter that if my wife actually ever had to drive the car their manual trans would be trash when they got it back assuming they didn't have to send a tow truck. Then all of a sudden I got a free upgrade from the compact automatic I had reserved too a mid sized automatic diesel. Ever since then I have leased. With a lease, a car is built for your order as opposed to a rental where you get whatever they have on hand. Make sure you specify the automatic when you make the reservation and be ready to stand your ground when you get there.

Thank you everyone for taking the time to reply. I realize a “reservation” and what is actually offered and available upon pick-up aren’t necessarily the same.

My take away is that it is possible to rent an automatic transmission.

I rent automatics when I have to drive on the left and have always received one. I pick up at an airport or major city and reserve well in advance.

I would add that if your trip is in May, to make your reservation asap. One rental agency told me that this year is going to be very busy and prices will probably go up. The sooner you reserve the more likely you will be able to score an automatic.

If you reserve an automatic the rental car company is obligated to give you an automatic. The “or similar” refers to makes, models, and size, not transmission. On one business trip I was on the rental car company had to get an automatic from another rental company. Years ago on a Scotland trip we reserved a compact automatic and we’re given a minivan. It was an automatic. One good thing is the cost difference between a manual and automatic isn’t as great as it used to be. For an upcoming trip the difference was only 30€ for a 20 days rental. Every time we’ve reserved an automatic we’ve gotten one. On our last trip to Span we got an automatic even when we reserved a manual.

HEllo - my wife and I are doing 10 days in Rep. of Ireland in October. After a week or so of somewhat confusing research, we booked our car rental with Budget just last week. Where and when I wanted to pick up and drop off appeared to be a significant factor in what company we dealt with. (Pick up Galway, drop off 4 days later at the Cork airport.) As far as I could tell, car rental in Galway was limited, but Budget has a location right near the train station. We made sure to reserve an automatic, just so we had one less thing to worry about. HOpefully we get it, but will survive if we don't! So, yes, you can get an automatic, but reserve early and be prepared to pay out the wazoo. Our 4-day rental is costing us just over $1000. (Including maximum insurance coverage.) Good luck!

Try using gemut.com for your car rental. They are my go to for all European car rentals. Super helpful people and always a great rate. Just used them for Ireland in September 2021 and for France last fall.