It is not a question of needing a passport to visit (you don`t) but as a means of identification for travel. You will find that all airlines require a form of positive I.D. for any flight - even within the U.K. so obviously a passport is the preferred choice. I do not know the current requirements if using a ship but it would better to be safer than sorry. These are fairly recent rules so if your trip to Dublin was a while ago then things have changed.
Eire is in the common travel area consequently passports are not REQUIRED but some form of photographic ID is required both by the airline and frequently by Special Branch officers upon return to UK
Everyone's right, 4 of us heading off there for a week from tomorrow. Passports are only needed for photo ID purposes. In 10 years of going golfing in Ireland, passports are only briefly looked at. If you've never been, you're in for a treat - hope you enjoy eating and friendly people.