Hey learner; welcome!
Ireland is one of my favorite places in Europe; you'll have a wonderful time.
I can't answer exactly how much gas costs there now, but as you probably know, it's very expensive in Europe!!! So be prepared to pay a lot....
I have never come across a hostel that doesn't have a shower anywhere in Europe, including Ireland. Granted, it might not be a very nice shower, but there should be one!
I found the most inexpensive way to eat in Ireland is by getting a few things at the local convenience store. They are franchises and all over the place, even in tiny villages out in the country. I can't remember the names of the two most prominent, but I remember the colors: either red or green! They are not like a convenience stores in the US where you'd just get a candy bar, they are like mini supermarkets. You can buy premade sandwiches, fruit, chips, cookies, whatever. If your hostel has a little kitchen, you can also buy things like lasagna or chicken and heat it up at the hostel for dinner. It's still usually more expensive than the US, but much cheaper than eating at restaurants all the time.
Hope this helps!
Ask if you have any other questions!