Tourists from some countries need valid Irish visas before entering Dublin, whether by plain, boat and ferry, or bus and car. An Irish entry visa is a certificate that is placed on your travel document or passport to show that you are permitted to visit Ireland.
Do you need a visa to enter the country? The Immigration Act 2004 (Visas) (No. 2) Order 2006 (S.I. No. 657 of 2006) has the complete list of countries whose people do not need a 
visa to land to Ireland.
Citizens of the European Economic Area (EEA) member states do not need visas in Ireland. The EEA members are the 27 countries of the European Union (EU), together with Liechtenstein, Iceland, and Norway. In addition, you may not need a visa to enter the country if you are an American, Canadian, or Mexican citizen.
But citizens from several countries require a transit visa when going to the country on their way to other country. Basically, a transit visa does not authorize you to leave the airport or port. A tourist will be needing a valid transit visa when landing to Ireland if he or she is a citizen of one of the following countries: Afghanistan, Albania, Cuba, Eritrea, Ethiopia, Ghana, Iran, Iraq, Lebanon, Moldova, Montenegro, Nigeria, Serbia, Somalia, Sri Lanka, and Zimbabwe.
You can get detailed information about the Irish visa application procedures on the Irish Naturalisation and Immigration Service (INIS) website. We advise that you browse and consult this official website before you apply for a visa. This is to ensure that you have the latest information. You can also see in this website a list of FAQs about visas in Ireland.