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).
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.
Are you a British or Irish citizen? Then you can do it, because of the Common Travel Area (Irish border authorities do conduct random checks, though). It is, however, strictly speaking illegal for somebody holding only a British visa to cross into the Republic, or vice versa.
Yes, but the Irish-British common travel area has been in existence since the instant there was a Republic. Our (Ireland's) visa policy on EU citizens is to all intents and purposes formulated in London because we're not going to have more than minimal border checks with the North and the UK isn't going to have internal border controls between the North and Britain.
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.)
Schengen is just one mutual agreement of no border controls between member countries and enforced border controls to other countries. Any 2 countries which are not part of Schengen are free to enter a similar bilateral agreement.
They may not be religious, but many borders (probably most of those which aren't due to the natural geography?) are the subject or result of some kind of conflict. So is this border really that special? Probably most borders in Europe were the subject of conflict within the last 100 years, but we've ended up with the Schengen agreement nevertheless.
