Along with staying at Air BnB places and/or hostels, I like to go to the market/grocery store and pick up stuff for sandwiches and snacks and wine/beer. You may or may not have access to a fridge but you could get a hard salami, bread and hard cheese and make a sandwich for lunch or dinner. For breakfast if I don't have a fridge I'll grab a yogurt from the grocery store along with a piece of fruit. (I buy several if I have a fridge to use.)
Depending on the kitchen at your Air BnB place you could also throw together a quick salad with tuna fish on top for an easy meal. Or there may be inexpensive foods you can snag from food carts or local cafes. (On my last trip to Spain we could get a decent breakfast for 2-3 euros so we did that most days.) I will definitely still eat out so I can experience the local food and culture, but I do it only occasionally or I'll do it on the cheap.
Also, museums typically have free or pay what you want days, so those are worth searching for. I think it's the first Sunday in a lot of European countries, not sure about Ireland though.
And refraining from buying a bunch of souvenirs helps keep costs down, too. I'd rather buy 1 special item if I find something I love, or just skip it altogether since I'll still have the memories.