Unless you are on an all-inclusive cruise you will need to pay for goods and services in Ireland. While cash is the most immediate form of payment and accepted everywhere, credit cards and traveler's checks can be an alternative.
When visiting Ireland you will have to cope with two currencies - the Republic is part of the Euro-zone while Northern Ireland uses Pounds Sterling. In the border regions both currencies tend to be accepted, but this should never be taken for granted.
One Euro () has 100 Cent (c) and coins are available in denominations of 1 c, 2 c, 5 c (all copper), 10 c, 20 c, 50 c (all golden), 1 and 2 (silver with gold). While the design of the side bearing the numerals is standardized throughout the Euro-zone the reverse is of local design, Irish Euros bearing a harp. Banknotes are totally standardized and most commonly available in denominations of 5, 10, 20 and 50. Higher denominations ( 100, 200 and even 500) are available but rare - some traders may refuse them.
One Pound Sterling (£) has 100 Pence (p) and coins are available in denominations of 1 p, 2 p (all copper), 5 p, 10 p, 20 p, 50 p (all silver), £ 1 (golden) and £ 2 (silver with gold). 50 c and £ 1 coins can have commemorative or local designs on the reverse. Banknotes are commonly available in denominations of £ 5, £ 10 and £ 20. £ 50 Notes are available but rare, again some traders may refuse them. Please note that banknotes are really issued by the individual banks, each using their own design - apart from notes issued by the Bank of England you will encounter notes from Northern Irish banks and the Bank of Ireland, you may also receive Scottish notes as change. All are valid currency but the different designs can be confusing.
Credit cards are widely accepted everywhere in Ireland with Visa and Mastercard being the most popular. Acceptance of American Express and Diners cards is decidedly lower. Most ATMs at banks will accept credit cards for cash withdrawal - but check the fees with your credit card company first. The actual exchange rate is quite good. Thus you should insist on being billed in Pounds Sterling or Euros when purchasing goods, not in Dollars (when the trader uses his own exchange rate).
Traveler's checks are an alternative to cash and credit cards but might not actually be accepted outside the major tourist centers.