To answer this and some of your other questions. There are many tips on this web site for "Budgeting in Ireland" and "Discounts" and "Free Coupons" just search on the INDEX provided by Michele. We took along our own peanut butter and jelly and bought beard at the grocery store in the Irish towns.
The drivers license question is also answered in the INDEX. US citizens do not need an international license. Don't know if it's better to pre pay for rental or not. We just put down a deposit to hold the rental car and pay up at the airport on arrival. Others here may know more about that question.
The euros vs dollars is also answered under Credit Cards and VAT in the INDEX. Look for the dynamic currency conversion explanation.
Most supermarkets have a section of pre-made sandwiches for around 3 euro or so. However, the ones with delis will make you a fresh sandwich with your choice of fillings for around the same amount. Pubs and restaurants have soup and brown bread (very filling) for perhaps 4 - 5 euro. Seafood chowder- that is a meal in itself - for around 6 - 7 euro. There are fast food places such as Super Macs, McDonald's, etc.
If staying at B&Bs the price includes a hearty breakfast, which can sustain you most of the day. A carvery lunch at many pubs generally costs about 10 euro and will give you a big plate of meat, potatoes, veg, etc. If you order coffee in a restaurant it only includes one cup. They don't have endless refills except for at breakfast in B&B and hotels. A pot of tea is a better value. A pint is generally about 5.50. Could be more in very popular big cities.
You don't have to have an entree at restaurants. You can just go in and order a "starter", which costs less. Many restaurants have online menus you can peruse or check out: http://www.menupages.ie/