Grocery stores in Ireland

It is my understanding every eatery in the Republic of Ireland is required to offer at least one vegetarian dish. It should be easy to adapt that to be vegan. I am a vegetarian and didn't have a problem anywhere finding something I could eat. I was there in 2018. Do you use Happy Cow? Try it out with some test cities. It has been a big help to me on several occasions. I just went to their website and see it is arranged by county, then town. There were 1687 listings for Ireland.

So as not to take any chances, maybe someone's personal item could be a Trader Joe's cooler bag? But yes, they sell the same type of thing over there. On such a short trip, I wouldn't want to spend time running around looking for one. Another option that is more packable is for each person to bring their own insulated lunch bag.
Most of those tourist towns will have vegan options in at least some of the restaurants. Here is a website that lists vegan restaurants by city: https://irishvegan.ie/vegan-food-restaurants-hotels/
OTOH, I doubt that grab-and-go sandwiches from a grocery store will be vegan (or very tasty, TBH--DIY is probably a better choice in Ireland).
This should go without saying, but with a car and a vegan in tow, a rental apartment is a better choice than a hotel.

When we traveled, we often dined a la grocery store. They have readymade salads, etc. There are usually sandwich shops everywhere. Look for Farmers Markets for Fruits and Veggies. And of course there are Bakeries. Indian Restaurants have lots of Veggie Dishes. No Costco yet. Besides saving money, you are saving time as you can eat picnic style.

I was in Dublin earlier this month solo. When I travel solo I stay in apartment hotels/Air BnBs so I can cook simple meals instead of eating out every meal. I found that many of the grocery stores in central Dublin were "city market" type stores that only sell prepared foods, drink and snacks and few if any actual groceries - I went in at least three Spar markets like this before I finally found a Dunnes that was an actual grocery store. I also went in a store I think just called "Fresh" that had a lot of prepared foods but a decent amount of groceries as well.
So if they are looking for groceries, they may need to venture into less touristed/more residential areas to find a full-service grocery store. However, they shouldn't have trouble finding vegan things like california rolls, rice bowls, tofu bowls, etc., either. Prepare sandwiches may be more problematic; they may be vegetarian but they are likely to have cheese on them.

North of the River Liffey in Dublin is the Jervis Shopping Centre, a fairly large mall. There's a Tesco there which might have what your friends are looking for. It's in the "real" Dublin, not tourist Dublin and a little on the gritty side. It is walking distance from the Temple Bar and Trinity College area. The only full service grocery store I know of for certain near Dublin is the Tesco just south of Ballsbridge at the Merrion Shopping Center. It wouldn't be convenient for Dublin but could be a slight detour depending on the route they plan to take upon leaving Dublin. My only other suggestion would be for them to go to Google maps and in the search bar type in grocery stores near whatever town they're in.

There are lots of Aldi and Lidl stores in Ireland, with selections similar to what Aldi has in the US. And Dunnes is an excellent option when you're in the larger cities. Think Marks & Spencer.

As for vegan and vegetarian dining, I wouldn't expect restaurant offerings to be very different in Dublin from, say, Louisville or Pittsburgh. Doolin and Dingle are likely a different matter.

ETA: a quick Google search finds many vegan restaurants in Dublin.

There’s also Marks and Spencer, also in Jervis Center, which is solidly in tourist Dublin.

Thank you all for the great suggestions! I will pass them along.
I think they are more concerned with finding food outside of Dublin since they will only be there one night. And they are staying at airbnbs or apartment rentals the whole time.
What about finding ice for the cooler? Do they have bags of ice at gas stations like here?
Thanks again for your input!

You don't buy ice for the cooler. Only Americans are obsessed with ice. You buy what you want/need at the shop and put it in the insulated bag. Every shop sells insulated, and regular, shopping bags. You will want both. You buy fresh every day.

Shopping the smaller markets, just like the big chains, is a daily thing. Just go in, get your stuff and then hit the road. Or stop at a place on the road. Many smaller towns will have shops that also sell hot food. Almost every shop will have fresh baked stuff. You want veg, buy veg. There's no need to over think this.

The chains are Lidl, Aldi, Tesco, and Eurospar.

I've never seen a restaurant that didn't list ingredients or offer a special menu. My wife is veggie and our travel companion is a celiac, and there's always good choices.

Unless they're headed to an uninhabited island AirBNB, there's always a local market within a few miles. Just shop for the next few meals and don't worry about a cooler. That way you support local markets and don't collect stuff you won't take home.

We did buy some whiskey glasses at a charity shop and liked them so much we brought them home, so there are exceptions.