UK nationals don’t need a passport to visit Dublin, but are advised to carry one (or some other form of photo identification) to prove that they are a UK national. It’s also necessary to have a passport or photo ID when changing travellers cheques or hiring a car. European Economic Area (EEA) citizens (that is, citizens of EU states, plus Iceland, Liechtenstein and Norway) can enter Ireland with either a passport or a national ID card. Visitors from outside the EEA will need a passport, which should remain valid for at least six months after their intended arrival.
For EEA nationals and citizens of most Western countries, including Australia, Canada, New Zealand and the USA, no visa is required to visit either the Republic or Northern Ireland, but citizens of India, China and many African countries do need a visa for the Republic. Full visa requirements for visiting the Republic are available online at www.dfa.ie.
Although you don’t need an onward or return ticket to enter Ireland, it could help if there’s any doubt that you have sufficient funds to support yourself while in Dublin.