Unless you are on an all-inclusive cruise you will, more than likely, need to pay at least for some for goods and services in Ireland. While cash is the most immediate form of payment and accepted everywhere (in fact preferred in a number of cases), credit cards and traveller's checks can be an alternative.
But there are pitfalls. When visiting Ireland you will, generally, have to cope with two currencies - the Republic is part of the Eurozone while Northern Ireland uses Pounds Sterling. In the border regions both currencies tend to be accepted, but this should never be taken for granted.
Republic of Ireland - Euro and Cents
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 Eurozone the reverse is of local design, Irish Euros bearing a harp. Non-Irish Euro coins are legal tender.
Take note that some machines will only accept non- Irish Euro coins with a bit of persuasion (try, try again) or not at all - Spanish coins are notorious in the latter department and can be a headache on automated tollbooths on the motorways.
Banknotes are totally standardized throughout the Eurozone 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. Improvements in design and paper quality have led to two versions of the € 5 note, the older one is accepted still but being phased out. September 2014 will also see the introduction of a new € 10 note.
Northern Ireland - Pounds and Pennies
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.
You should know that banknotes are issued by individual banks, not a central authority, each bank 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. The confusion is not eased by the takeover of Northern Bank, now part of Danske Bank, issuing Pounds Sterling with a Danish company name.
One word of caution - notes not issued by the Bank of England may be hard to exchange into your home country!
Possible - many shops in the border counties accept the "other" currency at their own (sometimes quite favourable) exchange rate. You will, however, only get change in the local currency. You'll also find the odd parking metre that will accept Euros in Northern Ireland.
Plastic is Fantastic
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. There might also be a minimum purchase clause in many shops - no credit card transactions below € 10 or even £ 20.
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 (usually) 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).
Credit card skimming is on the decline, but still a risk.
In Northern Ireland, only credit cards using the "chip and PIN" system are accepted in shops. In the Republic, things are heading that way too.
Debit cards are also widely accepted, again you should check with your card provider before travelling.
Old School - the Cheque
Traveller's checks used to be a secure and convenient alternative to cash and credit cards ... but even historically were not actually accepted outside the major tourist centres. These days, they are definitely facing extinction. Most traders will not accept them any more, and you will even have problems exchanging them in most banks.
Personal cheques are, generally speaking, not accepted at all.