I cross the border between the Republic of Ireland (independent) and Northern Ireland (part of UK) in a car all of the time, never stopped once despite lack of Schengen.
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.)
http://en.wikipedia.org/wiki/Segregation_in_Northern_Ireland