Schengen Agreement, which eliminates border control is in effect in 26 countries (including non-EU countries, like Iceland, Norway and Switzerland, but excluding some EU countries, for example UK).
http://en.wikipedia.org/wiki/Segregation_in_Northern_Ireland
Not as weird as the fact that Irish people without UK citizenship, resident there can vote in all elections, and that UK citizens resident in Ireland can vote in all elections bar for President (seeing as Irish people in the UK don't get to vote on the Head of State.)