Originally Posted by
paul4040
If you are a citizen of the UK or Republic of Ireland who was born in that country you do not need a passport to travel between the two countries but you do require some form of photographic identification, such as a driving licence.
Just to add emphasis to this - you need to be born in either the UK or Ireland to travel on a driving licence (which does actually show this). Otherwise, even if you are a naturalised UK or Irish citizen, you need to present a passport.
The joys of the CTA!