Think you don't need a passport to travel between the UK and Ireland? If you ask Aer Lingus, the Irish Embassy in London or the government they say you don't. If you travel from Ireland to the UK you'll never be asked for a passport.
But if you try to return you risk arrest.
I was detained in Cork airport because I flew from Heathrow with a photo-ID only. Aer Lingus let me on the plane.
Initially I assumed the Garda was just off his rocker. but I researched it and found out that there is a dispute between the GNIB and the government on the issue. The GNIB reserve the right to demand passports from all people entering the state, while the government continues to advise the airlines and travellers that only photo-ID is needed.
Ireland and the UK have an arrangement called the Common Travel Area which is supposed to allow Irish or British citizens to travel without passports. Britain respects the arrangement, and flights from the Republic land at the domestic sections of UK airports. No-one asks for a passport. The arrangement doesn't apply to non-UK or Irish citizens. But this is ignored in the UK.
I had a question asked in the Dail of McDowell last March. He answered that while Irish and UK citizens don't need a passport to enter the state, they do need to prove their nationality. The only acceptable proof is a passport. (What a wonderful example of McDowell legalese fudge!) To date the government websites have not been updated - they still say only photo-ID is required.
Why the confusion? Why the mystery?
I found that out too. In the Amsterdam Treaty, Ireland got an opt out from the Schengen Treaty only while the Common Travel Area exists. If the EU got wind of the fact that the CTA has been quietly done away with, Ireland would be legally obliged to join Schengen.