The fact that there is no border control does not free travellers from holding proper travel documents.
• British and Irish citizens may use one of the following documents for travelling in the Common Travel Area
- a valid passport
- a driver's licence with photo
- an international student card
- a national ID card
- a bus pass with photo
- a Garda ID with photo
- a work ID with photo
• other EU/EEA citizens [except Danish] and citizens of Switzerland may use their national ID cards or a passport
• Danish citizens need the passport
• other citizens will need the passport and maybe, a U.K. visa
http://www.citizensinformation.ie/catego… ..
http://www.ukvisas.gov.uk/en/doineedvisa… ..
Edit: based on other contributions
EU citizens do not need a passport to travel in the EU to include Ireland and the UK if they hold a valid national ID card.
http://bia.homeoffice.gov.uk/travellingt… [read: what travel documents are accepted at the border]
At present citizens of 60 non-EU countries (f.e. America, Australia, Canada, New Zealand) may enter the UK without visa!