Whether you need a passport to go to Dublin from the UK depends on your citizenship. This state of affairs relates to a freedom-of-travel agreement enjoyed by some citizens. The UK Government offers advice about passports and other forms of ID with specific reference to Ireland.
Dublin is a popular destination, being a cultural city of immense charm. Whether you want to wallow in the literary scene or enjoy traditional Irish dancing and ales, Dublin has something to offer. A flight to Dublin airport takes only about an hour from many parts of the UK. Sea crossings take longer but are often less expensive.
Common Travel Area
The Common Travel Area (CTA) consists of The United Kingdom, The Republic of Ireland, The Bailiwick of Guernsey, The Bailiwick of Jersey and the Isle of Man. UK and Irish citizens may travel freely in the Common Travel Area without a passport. This arrangement is referred to in the Treaty of Amsterdam. Therefore, you don’t need a passport to go to Dublin from the UK if you are a UK or Irish citizen.
UK Government advice
If you travel by air to Dublin from the UK, the UK Government advises that you should carry proof of nationality. Further, many passenger carriers require travellers to provide photo ID. Since a passport is a very reliable proof of nationality, and also provides photo ID, it is sensible to carry your passport with you -- if you have one -- when travelling to Dublin from the UK.
Non-UK and non-Irish citizens
Non-UK and non-Irish citizens require a passport to travel to Dublin from the UK. This is because they are not beneficiaries of the Common Travel Area arrangements. If you are a non-UK and non-Irish citizen, you may be asked to produce your passport when travelling to Dublin from the UK. Failure to produce your passport on request may result in your being unable to proceed any further than your intended departure point.