Do you need a passport to travel from dublin to the UK?

As the asker of this question did not state his or her nationality the best advice anyone should give is to check with the Immigration Control authorities of both the Republic of Ireland and the United Kingdom at least a week BEFORE the day you plan to travel between London and Dublin. If you are a citizen of the United Kingdom you don't strictly need a valid passport to travel between the UK and Ireland. It's likely, though, that your airline or sea ferry service will require you to produce some other form of photographic id. If you are a citizen of any other country which is a Member State of the European Union it may or may not be essential to have a valid passport if you can produce some other form of photographic id which is acceptable to the Immigration Control authorities of both the Republic of Ireland and the United Kingdom. However, as traveling without a valid passport may lead you to experience delays at Immigration Control when entering the Republic of Ireland and again when you re-enter the United Kingdom, it would be very sensible and wise of you to use a valid passport to travel between the UK and Ireland.

If you are a citizen of any country that is outside the European Union you will need to produce a valid passport and also - if you are a citizen of certain countries - a valid visa to enter the Republic of Ireland.