firstly i'd talk to the chef at the place where you are staying, if its a hotel or bed and breakfast. if you're staying self catering, look online to see what supermarkets are near to where you are staying, or that you can pop into on your way to your location. i've found that if you do your homework and find out what big branches of irish supermarkets are in the area, -and look online to see what vegan food basics they have, like rice and soy milk, and margarine, and can give the chef (or have prepared for yourself if you are staying self catering) a list of things you CAN have, and where he/she/you can get these items, people often really rise to the challenge and create you some lovely vegan meals. (a lot of chefs have told me they appreciate the excuse to do something different when they do speciality meals!)
i'm from england, but have stayed in hotels, hostels, and bed and breakfasts in rural parts of the UK, and as well as being vegan i have a whole wodge of food allergies and intollerances, so i know it can be done, if you prepare. ireland has really similar supermarkets and products to us in the UK, (look online for things you can get- i know they have 'tesco ireland' for a start- a big chain supermarket- you can use their 'shop online' function to find out what they have that suits you like soy milk, etc.
although you'll find a lot less in the way of pre made vegan sausages and burgers, etc, they will no doubt have basics, like suitable kellogs type breakfast cereals, peanut butter, jam, heinz baked beans in tomato sauce (no animal fat) canned pulses, store own brand, provomel and rice dream rice and soy milk (and maybe even soy yogurts) cauldron tofu, and vegan margarine (probably supermarket generic soy margarine), etc, and there should be loads of fresh fruit and veggies on offer.
from what friends have told me, irish food is pretty heavy on 'meat, veg and bread, and they use a lot of potatoes. eating loads of veggies for a few weeks can be boring, but it won't hurt you long term!
i've found it pretty easy to arrange to switch things a little from the set menu in most places i've visited, especially if i can provide alternatives (or tell my hosts where they can get them!) in advance. if it comes down to it, a basic menu of things like fruit, cereal with rice or soy milk, or baked beans on toast for breakfast, salad (or even peanut butter and jam!) sandwiches, mixed salad, a baked potato and beansalad for lunch, and veggies with pasta and tomato sauce, veggie curry, veggie stirfry, vegetable soup etc, for dinner for a few weeks, with dried fruit and nuts to snack on, won't be the most exciting, but it won't kill you (though by the end you might wish it had, lol!)
the main things as usual to be clear about when talking to whoever will be cooking for you, is that your food isn't cooked in or with meat fat or butter (still quite common in europe- especially with pastry) and to ask for any eggs to be omitted, and milks and butters to be switched to your versions. most importantly, let them know what these versions are, and where to get them. (most non vegans i've met in the uk don't know about much about soy and rice milks or vegan margarines, or where to get them- despite them being right there on the shelf in the supermarket that they visit for their weekly shop.
you could also consider buying from a big supermarket, or even better trying to borrow from your hosts, a cheap mini fridge (those ones for plugging in to the car that hold about 6 cans- don't take one with you, lol- we're on 240 volts!) to keep in your room for your own little stash of soy yogurts, soy milk, margarine, snacks, etc, if you hit a brick wall with your hosts, this way you can eat what you can from what they have, but top up on your own stuff, and use a kettle from your room or hot tap to make up some 'add water' things like prepacked soups and hot drinks.
as for eating out in pubs, i'm sorry, but from experience i've found its not likely to be amazing. i've usually been limited to either a plain baked potato with side salad and ketchup or baked beans, or a salad, or a cheese ploughmans salad minus the cheese, or fries (we call 'em chips!) and  onion rings, if i'm lucky and they haven't been cooked in meat fat,  salad sandwiches, or side orders of mixed boiled veggies and plain rice. some places do suprise you with a really tasty veggie burger on the menu though!
as for bringing food into the country- i'd be careful what you bring in, as customs might seize it and tell you off if you get it wrong (that's if you mention it to them, or don't say you need them as you have food allergies). sealed packets of cookies and crackers should be ok, as should a few cartons of soy or rice milk, and 'add water' things like soup, hot chocolate, noodles, dried stock, etc if you want to bring them as an emergency stockpile or backup, incase you can't get to anywhere to buy your own for a while. you should really be able to get most really basic basics like peanut butter and canned (yuck) veggies etc in even a corner store, though. let me know if i can help any more- i'll give it a shot, and sorry for the long post!
ireland is a really beautiful place, and the people are very very friendly (watch out not to swoon over the charming men with their gorgeous accents, lol!), so having to cope with a few weeks of eating a lot of veggies isn't too bad a payback for what you'll experience.
hope you have a wonderful time!