Do you need a valid UK passport when visiting southern Ireland?

When travelling by Ferry from UK no identification as such is required. If you are not an EU member it would be advisible to bring some. No passport is required.

By Air from UK: All flights now require photo identification: Passport or new style Driving licence is acceptable (about the only two).

no it is part of the EU

Depends which airline you fly with, Ryanair used to accept photo id like a driving license but i think this has changed, but you need to check the websites of the airline your flying wiht to make sure.

when traveling between ireland and the uk you just need a photo id
no passport
they have a deal or something

No you don't. There has always been free passage between Britain and The Republic of Ireland.

no just photo id

If you are travelling by ryan air you need to have a valid passport, also if you choose to fly with them ring and let them know how many pieces of luggae you are going to check in as there is a fee now perbag. Im not sure of the cost but I know its less if you inform them. Aer Lingus/British Midland state you need photographic ID ie passport, driving license.

if you travel by most plane company`s you do, jet be take photo id signed by police. I TRAVEL BY BOAT MOST OF THE TIME AND DON`T NEED PASSPORT JUST AS LONG AS YOU SHOW SOME ID