Common Travel Area

The United Kingdom and Ireland are not part of the Schengen agreement that makes it possible to travel through most of Europe without passport checks at every border.  Instead they have the Common Travel Area. 

The CTA is basically a mini-Shengen agreement that allows for open travel between Ireland and the UK, as well as, the British Crown dependencies of the Isle of Man, Jersey, and Guernsey.  Sort of.

Since 1997 Ireland checks everyone arriving by airplane.  So when I land in Dublin it doesn't matter if I flew from London or Prague, I still have to go through immigration and get my passport stamped. 

However, when I've flown from Dublin to London Standsted, London Gatwick and Inverness there was nothing.  No immigration or passport check.  There are random checks though.  When I flew from Dublin to Cardiff there was a random check.  I had to show my passport but it wasn't stamped.  I guess because I was already in the CTA. 

One of the confusing things for me is that Ireland and the UK each have different visa requirements.  So it's possible for someone to need a visa for one of the countries but not the other.  Yet, the CTA would allow the person to travel freely (except for a random check).

If either country ever joined Schengen then the CTA would end because of the requirement to secure borders with non-members.  I've heard that Ireland would like to join Schengen but I don't see that happening.  The border between Ireland and Northern Ireland is open.   If Ireland joined Schengen then the border between the two countries would have to be closed and it hasn't been that long since "the Troubles" ended.  Any hopes for a united Ireland would immediately end.