For jimmie from the Irish Government website.
"Rules

The Common Travel Area means that there are no passport controls in operation for Irish and UK citizens travelling between the 2 countries. You do not need to have a passport in order to enter the other country. However, all air and sea carriers require some form of identification and some regard a passport as the only valid identification. Immigration authorities may also require you to have valid official photo-identification which shows your nationality.

As you are being asked to prove that you are an Irish or UK citizen who is entitled to avail of the Common Travel Area arrangements, it is advisable to travel with your passport.