Looks like most of what you need has been covered, but I want to add my two cents, having been to Ireland eight times and also being budget minded.
While the large supermarkets are indeed usually not in the center of town, it's worth making a trip to one when you have the car. You (and your kids!) will be amazed at the selections of cookies, cakes, chocolates, etc. Rather than buying sandwiches, we usually buy sliced meats, cheese, butter and the really good fresh bread they have. Many will have salad bars, with various cold foods to take out. Bring an insulated carrier that folds flat, and keep it next to the window in your hotel room- mini fridge! We've found this keeps cheese, butter, etc, very well for a couple of days. Bring a serrated knife (in your checked bag!), for bread and cheese slicing. Reusable plates, or just paper towels, and zip lock bags. It shouldn't be any problem finding breakfast and lunch foods to eat in your room. And even in places where there is no large supermarket, the smaller shops usually have good bread, cheese, yogurt, etc. You will want to be subtle bringing this to your room, as some places frown on eating in the room, although as mentioned above, everyone is very nice, so no one is going to berate you. Just be considerate of crumbs, chocolate, etc, of course.
Also if you are into cooking, it's a lot of fun to just look around the large supermarkets in a foreign country to see the different foods available. We usually bring back goodies for souvenirs, and bring back tea/coffee that we can't get here for ourselves.
You will have a great time! I also look forward to a trip report!