Citizens of some countries require a visa for entry into Ireland. Visas issued for the United Kingdom are NOT valid for the Republic of Ireland; a separate Irish visa is required. Visas can be applied for at Irish Embassies and consulates abroad. For more details check with the Department of Foreign Affairs. There is a lot of information on visa requirements and how to apply on this website:

Arriving from Great Britain (mainland UK): As part of the Common Travel Area, British and Irish citizens are not formally required to have passport. They should, however, have some form of identification available to prove their nationality (such as a passport, birth certificate, etc). In practice, it is sensible for British and Irish citizens to have a passport. Citizens of all other countries must produce a valid passport (and a visa if necessary). Officers of the Garda National Immigration Bureau will check passports/identity papers at airports and ferry ports.

Arriving from Northern Ireland: no passport controls are enforced. The border between the Republic of Ireland and Northern Ireland is open and barely discernable. Normally no passport is needed when driving or travelling by train or bus from one into the other. If you require a visa for either Ireland or the UK you must carry your passport with you, with the appropriate visa, when crossing the border. Citizens of countries requiring visas for travel to Northern Ireland should consult the UK Border Agency. If driving across the border, check that your vehicle insurance is valid and ensure that you have your driving licence with you.

In Northern Ireland speed limits and distances will be shown in miles (kilometres in the Republic). Northern Ireland uses pounds sterling, the currency in the Republic is Euro.

British Citizens when travelling to Ireland do not require passports, but they should carry identification. For citizens of the EU, Norway and Switzerland, the national identity card is enough (no passport required). All visitors from other countries must carry a valid passport.

Arriving from all other countries: Most travellers will arrive at Dublin, Cork or Shannon Airports, or Cork or Rosslare ferry terminals. Ireland is not part of the Schengen passport-free travel area. All travellers will need to show a valid passport on arrival (or national identity card for citizens of EU/EEA member states) and a visa if necessary.